Multilayer infrared reflecting optical body

ABSTRACT

An optical body comprising (a) a dielectric multilayer film having a reflecting band positioned to reflect infrared radiation of at least one polarization at an incident angle normal to the film, the reflecting band having a short wavelength bandedge λ a0  and long wavelength bandedge λ b0  at a normal incident angle, and a short wavelength bandedge λ aθ  and long wavelength bandedge λ bθ  at a maximum usage angle θ, wherein λ aθ  is less than λ a0  and λ a0  is selectively positioned at a wavelength greater than about 700 nm; and (b) at least one component which at least partially absorbs or reflects radiation in the wavelength region between λ aθ  and λ a0  at a normal angle of incidence.

This is a divisional of application Ser. No. 09/005,727 filed Jan. 13,1998.

BACKGROUND OF THE INVENTION

The reduction of solar heat load entering a building or vehicle throughits windows is important in minimizing air conditioning load andpromoting personal comfort. Clear infrared rejecting films have beenmade using metalized or dyed polymer films and multilayer polymer stacksthat reflect or absorb unwanted infrared radiation. Ideally, such filmstransmit all light in the wavelength region sensitive to the human eye,typically from about 380 to about 700 nanometers (nm), and reject solarradiation outside the visible portion of the spectrum. Metalized anddyed polymer films suffer from reduced performance when used forextended periods of time in window film applications, as they aresusceptible to UV degradation and chemical assault from various sources.Their. failure mechanism is typically non-uniform, creating poor visualappearance after prolonged exposure. Also, the reflectivity of metallayers originates from a thin coating and if this coating is damaged,the performance of the film is decreased. Clear infrared rejecting filmcan be made from a quarter wave mirror that has its reflecting band inthe near infrared. Infrared rejecting films made from alternating layersof metal oxides have been described in U.S. Pat. No. 5,179,468, U.S.Pat. No. 4,705,356 and EP 0 080 182. Films made from a combination ofmetal and metal oxide layers have been described in U.S. Pat. Nos.4,389,452; 4,799,745; 5,071,206; and 5,306,547. Infrared rejecting filmsmade from alternating layers of polymers with high and low indices ofrefraction have been described in U.S. Pat. Nos. RE 34,605; 5,233, 465;and 5,360,659; U.S. Ser. Nos. 08/402,041 entitled “Optical Film” and08/67;2691 entitled “Transparent Multilayer Device; and U.S. Ser. No.09/006,118 entitled “Multicomponent Optical Body”, filed on even dateunder Attorney Docket No. 53543USA1A, in which a generalized scheme isdescribed for controlling higher order reflections while maintainingdesired relationships between the in-plane and out-of-plane indices ofrefraction so that the percent reflection of the first order harmonicremains essentially constant, or increases, as a function of incidenceangle. These films are not susceptible to the same degradationmechanisms as thin metal or metal oxide layers or dyed films, as it isnecessary to destroy the entire film to reduce performance. The filmsare highly corrosion resistant, have a neutral color, and can havevarious properties built into the film, such as antistatic, abrasionresistant, and slip layers incorporated in the film's surface. Theflexibility and manufacturing cost of the films make them well suitedfor use as a laminate to glass before window construction as well as forretrofit applications.

For many applications, it is desirable that the infrared reflective filmreflect as much solar radiation as possible in the infrared portion ofthe spectrum while maintaining essentially complete transparency in thevisible region of the spectrum.

One problem with the quarter-wave polymeric films is that without propercompensation to eliminate overtones, higher order reflections willappear at fractions of the first order reflection and exhibitiridescence and visible color. Mathematically, higher order reflectionswill appear at

λ_(m)=(2/M)×D_(r)

Where M is the order of the reflection (for example, 2, 3, 4, etc.) andD_(r) is the optical thickness of an optical repeating unit, of whichmultiple units are used to form the multilayer stack. Accordingly, D_(r)is the sum of the optical thicknesses of the individual polymer layersthat make up the optical repeating and the optical thickness is theproduct of n_(i), the in plane refractive index of material i, andd_(i), the actual thickness of material i. As can be seen, higher orderreflections appear at fractions of the first order reflection. Forexample, a film designed to reflect infrared radiation between about 700and 2000 nm will also reflect at 1000 nm, 667 nm, 500 nm, of which thelatter two are in the visible range and would produce strong iridescentcolor. It is possible to suppress some higher order reflections byproper selection of the ratio of the optical thicknesses in twocomponent multilayer films. See, Radford et al, “Reflectivity ofIridescent Coextruded Multilayered Plastic Films”, Polymer Engineeringand Science, vol. 13, No. 3, May 1973. This ratio of optical thicknessesis termed “f-ratio”, for a two component film, where f=n1 d1/(n1 d1+n2d2). Such two component films do not suppress successive second, thirdand fourth order visible wavelengths. Optical coatings comprising layersof three or more materials have been designed which are able to suppresscertain higher order reflections. For example, U.S. Pat. No. 3,247,392describes an optical coating used as a band pass filter reflecting inthe infrared and ultraviolet regions of the spectrum. The coating istaught to suppress second and third order reflectance bands, but thematerials used in the fabrication of the coating are metal oxide andmetal halide dielectric materials which must be deposited in separatesteps using expensive vacuum deposition techniques. Other vacuumdeposition techniques used to reduce higher order reflections are taughtin U.S. Pat. Nos. 3,432,225 and 4,229,066, and in “Design of Three-LayerEquivalent Films”, Journal of the Optical Society of America, Vol. 68(I), 137 (January 1978). U.S. Pat. No. RE 34,605 describes an allpolymeric three-component optical interference film formed bycoextrusion techniques which reflects infrared light while suppressingsecond, third and fourth order reflections in the visible region of thespectrum. The polymers in the film are required to have closely definedrefractive indexes, which limits the choice of polymers which may beused, and production of the film requires separate extruders for each ofthe three polymeric components. U.S. Pat. No. 5,360,659 describes an allpolymeric two-component film which can also be coextruded and reflectsinfrared light while suppressing second, third, and fourth orderwavelengths which occur in the visible portion of the spectrum. The filmcomprises alternating layers of first (A) and second (B) diversepolymeric materials having a six layer alternating repeat unit withrelative optical thicknesses of about 7:1:1:7:1:1 for the layers ofA:B:A:B:A:B, respectively. In an alternative embodiment of the inventionthe two-component film comprises a first portion of alternating layerscomprising the six layer alternating layer repeating unit with relativeoptical thicknesses of about 7:1:1:7:1:1 for the layers of A:B:A:B:A:B,respectively, and a second portion of alternating layers having arepeating unit AB of equal optical thicknesses.

A second problem with the quarter-wave polymeric films, or anydielectric reflectors, is that the reflection band shifts in wavelengthwith observed incident angle. When this happens, there is a dramaticcolor change at high angles of incidence, with cyan color observed inreflection and deep red observed in transmission. The shift in thereflection band is caused by the change in effective index of refractionwith angle. Both the band centers and the width of the reflection bandchange as the incidence angle changes, with the reflecting band alwaysshifting towards shorter wavelengths. This is counterintuitive, as thetotal path length increases with angle. The band position does notdepend on the total path length, but the difference in path lengthbetween reflections off the interfaces, and this difference decreaseswith angle. The high wavelength bandedge also shifts differently fromthe low wavelength bandedge. For low wavelength bandedges, the change incenter and width with angle tend to cancel. For the high wavelengthbandedge, the changes add to broaden the band. Typically, for thematerials under consideration, a bandedge shifts to about 80% of itsnormal incidence wavelength when viewed at grazing incidence. For thepresent applications, in order to not have visible color when the filmis viewed at non-normal angles, it is necessary that the low wavelengthbandedge of an infrared reflector be positioned sufficiently far intothe infrared so that it is not observed at a desired use angle.Typically, the film must be designed so that the short wavelength edgeof the normal angle is shifted 100-150 nm away from the edge of thevisible spectrum. For example, for a multilayer infrared reflecting filmhaving alternating layers of PEN and PMMA, the short wavelength bandedgemust be moved to about 850 nm to eliminate any perceived color withangle. This creates a gap between the edge of the visible spectrum(about 700 nm) and the low wavelength bandedge of about 150 nm.

For many applications it is desirable to reflect as much of the solarspectrum as possible which contributes to heat load, while transmittingall of the visible spectrum. Shifting the bandedge to longer wavelengthsfor the normal incidence condition results in a gap between thereflectance band and the visible edge of the spectrum, resulting inlower spectral coverage at wavelengths where the solar infrared spectrumis a maximum. This correlates to an over-all increase in the shadingcoefficient of the film, which is a measure of the amount of solarenergy that enters the window compared to that of a simple pane ofglass. Accordingly, the need exists for an infrared film that reflectsthe maximum amount of solar infrared even when the reflecting band isshifted to compensate for visible color when the film is viewed atnon-normal angles. It is further needed that such a film be able toeliminate higher order reflections that also contribute to visible colorand iridescence.

U.S. Pat. No. 5,486,949 discloses that it may be desirable toincorporate coloring agents such as dyes or pigments into one or morelayers of a birefringent polarizer to permit selective absorption ofcertain wavelengths of light and control the bandwidth of reflectedpolarized light and the wavelength range of transmitted light. U.S. Pat.No. 4,705,356 discloses a thin film optically variable article havingsubstantial color shift with varying angle of light incidence andviewing comprising an optically thick substantially transparentstructural element carrying a colorant and a multilayer interferencecoating, whereby the colorant serves to modify in essentially asubstractive mode the color at normal incidence and the color shift withangle of the multilayer interference coating as seen by transmission oflight through the article. Neither U.S. Pat. No. 5,486,949 nor U.S. Pat.No. 4,705,356 disclose an optical body comprising a film having areflecting band positioned to reflect infrared radiation of at least onepolarization at an incident angle normal to the film combined with acomponent designed to at least partially absorb or reflect infraredradiation at normal incidence in the region resulting from thepositioned reflecting band.

SUMMARY OF THE INVENTION

The present invention relates to an optical body comprising (a) abirefringent dielectric multilayer film, which may be a polarizer,mirror, or both, having a reflecting band positioned to reflect infraredradiation of at least one polarization at an incident angle normal tothe film, said reflecting band having a short wavelength bandedge λ_(a0)and long wavelength bandedge λ_(B0) at a normal incident angle, and ashort wavelength bandedge λ_(aθ) and long wavelength bandedge; at amaximum usage angle θ, wherein λ_(aθ) is less than λ_(a0) and λ_(a0) isselectively positioned at a wavelength greater than about 700 nm; and(b) at least one component which at least partially absorbs or reflectsradiation in the wavelength region between, λ_(aθ) and λ_(a0) at anormal angle of incidence.

The present invention also relates to an optical body comprising (a) anisotropic dielectric multilayer film having a reflecting band positionedto reflect infrared radiation of at least one polarization at anincident angle normal to the film, said reflecting band having a shortwavelength bandedge λ_(a0) and long wavelength bandedge λ_(b0) at anormal incident angle, and a short wavelength bandedge λ_(aθ) and longwavelength bandedge λ_(b0) at a maximum usage angle θ, wherein λ_(aθ) isless than λ_(a0) to and λ_(a0) is selectively positioned at a wavelengthgreater than about 700 nm; and (b) at least one component which at leastpartially absorbs or reflects radiation in the wavelength region betweenλ_(aθ) and λ_(a0) at a normal angle of incidence.

The optical body of the present invention provides good reflectivity inthe infrared region of the spectrum and improved shading coefficient atnormal angles while still transmitting visible light at all desirableangles of incidence.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained with reference to the drawings,in which like numbers designate like structures throughout the variousFigures, and in which:

FIG. 1 is a schematic illustration of the effect of a multilayer film ofthe present invention when viewed by an observer at two points relativeto the film;

FIG. 2 is a perspective view of a multilayer film useful in the opticalbody of the present invention;

FIGS. 3-6 are transmission spectra associated with various actual andmodeled film samples;

FIG. 7 is a schematic diagram of a manufacturing process for making amultilayer film useful in the optical body of the present invention.

FIG. 8 is a reflectance spectrum showing a shift in the reflectivityband with angle.

DETAILED DESCRIPTION OF THE INVENTION

The infrared film of the present invention can be designed so that theshort wavelength edge of the normal angle spectrum is a certainwavelength, depending on the requirements of the end application, awayfrom the edge of the visible, for example, 100-150 nm away. This allowsthe film to be designed to avoid off-angle color changes, for example,the film may be designed so that the off angle shift does not allow thelow wavelength bandedge to encroach into the visible and cause color or,when there is already visible color at normal angles, the film can bedesigned so that the off angle color shift does not cause perceptiblecolor change in the film. Shifting the bandedge to longer wavelengthsfor the normal incidence condition results in lower spectral coverage atwavelengths where the solar infrared spectrum is a maximum. In thepresent invention, a wavelength gap filler component is used to cover atleast a part of the gap between, for example, the short wavelengthreflecting bandedge and the edge of the visible spectrum.

Film

The film of the present invention comprises at least two layers and is adielectric optical film having alternating layers of a material having ahigh index of refraction and a material having a low index ofrefraction. The film may be isotropic or birefringent. Preferably, thefilm is a birefringent polymeric film, and more preferably the film isdesigned to allow the construction of multilayer stacks for which theBrewster angle (the angle at which reflectance of p polarized light goesto zero) is very large or is nonexistent for the polymer layerinterfaces. This feature allows for the construction of multilayermirrors and polarizers whose reflectivity for p polarized lightdecreases slowly with angle of incidence, is independent of angle ofincidence, or increases with angle of incidence away from the normal. Asa result, the multilayered films of the present invention have highreflectivity (for both s and p polarized light for any incidentdirection in the case of mirrors, and for the selected direction in thecase of polarizers) over a wide bandwidth. The film of this inventioncan be used to prepare multilayer films having an average reflectivityof at least 50% over at least a 100 nm wide band in the infrared regionof the spectrum. filed Suitable films include those described in U.S.Ser. No. 08/402,401 filed Mar. 10, 1995, and U.S. Ser. No. 09/066,601,entitled “Modified Copolyesters and Improved Multilayer Reflective Film”filed on even date under Attorney Docket No. 53550USA6A”, both of whichare hereby incorporated by reference.

Suitable films also include those which prevent higher order harmonicswhich prevent color in the visible region of the spectrum. Examples ofsuch film include those described in U.S. Pat. No. RE 3,034,605,incorporated herein by reference, which describes a multilayer opticalinterference film comprising three diverse substantially transparentpolymeric materials, A, B, and C and having a repeating unit of ABCB.The layers have an optical thickness of between about 0.09 and 0.45micrometers, and each of the polymeric materials has a different indexof refraction, ni. The film includes polymeric layers of polymers A, B,and C. Each of the polymeric materials have its own different refractiveindex, n_(A), n_(B), n_(C), respectively. A preferred relationship ofthe optical thickness ratios of the polymers produces an opticalinterference film in which multiple successive higher order reflectionsare suppressed. In this embodiment, the optical thickness ratio of firstmaterial A, f_(A), is ⅕, the optical thickness ratio of second materialB, f_(B), is ⅙, the optical thickness of third material C, f_(c) is ⅓,and n_(B)={square root over (n_(A)n_(C))}.

For this embodiment, there will be an intense reflection at the firstorder wavelength, while the reflections at the second, third, and fourthorder wavelengths will be suppressed. To produce a film which reflects abroad bandwidth of wavelengths in the solar infrared range (e.g.,reflection at from about 0.7 to 2.0 micrometers), a layer thicknessgradient may be introduced across the thickness of the film. Thus, thelayer thicknesses may increase monotonically across the thickness of thefilm. Preferably, for the preferred three component system of thepresent invention, the first polymeric material (A) differs inrefractive index from the second polymeric material (B) by at leastabout 0.03, the second polymeric material (B) differs in refractiveindex from the third polymeric material (C) by at least about 0.03, andthe refractive index of the second polymeric material (B) isintermediate the respective refractive indices of the first (A) andthird (C) polymeric materials. Any or all of the polymeric materials maybe synthesized to have the desired index of refraction by utilizing acopolymer or miscible blend of polymers. For example, the secondpolymeric material may be a copolymer or miscible blend of the first andthird polymeric materials. By varying the relative amounts of monomersin the copolymer or polymers in the blend, any of the first, second, orthird materials can be adjusted so that there is a refractive indexrelationship where n_(B)={square root over (n_(A)n_(C))}.

Another suitable film includes the film described in U.S. Pat. No.5,360,659, incorporated herein by reference, which describes a twocomponent film having a six layer alternating repeating unit suppressesthe unwanted second, third, and fourth order reflections in the visiblewavelength region of between about 380-770 nm while reflecting light inthe infrared wavelength region of between about 770-2000 nm. Reflectionshigher than fourth order will generally be in the ultraviolet, notvisible, region of the spectrum or will be of such a low intensity as tobe unobjectionable. The film comprises alternating layers of first (A)and second (B) diverse polymeric materials in which the six layeralternating repeat unit has relative optical thicknesses of about.778A.111B.111A.778B.111A.111. The use of only six layers in the repeatunit results in more efficient use of material and simpler manufacturethan previous designs. A repeat unit gradient may be introduced acrossthe thickness of the film. Thus, in one embodiment, the repeat unitthicknesses will increase linearly across the thickness of the film. Bylinearly, it is meant that the repeat unit thicknesses increase at aconstant rate across the thickness of the film. In some embodiments, itmay be desirable to force the repeat unit optical thickness to doublefrom one surface of the film to another. The ratio of repeat unitoptical thicknesses can be greater or less than two as long as the shortwavelength range of the reflectance band is above 770 nm and the longwavelength edge is about 2000 nm. Other repeat unit gradients may beintroduced by using logarithmic and/or quartic functions. A logarithmicdistribution of repeat unit thicknesses will provide nearly constantreflectance across the infrared band. In an alternative embodiment, thetwo component film may comprise a first portion of alternating layerscomprising the six layer alternating layer repeating unit which reflectsinfrared light of wave lengths between about 1200-2000 nm and a secondportion of alternating layers having an AB repeat unit and substantiallyequal optical thicknesses which reflect infrared light of wavelengthsbetween about 770-1200 nm. Such a combination of alternating layersresults in reflection of light across the infrared wavelength regionthrough 2000 nm. Preferably, the first portion of the alternating layershas a repeat unit gradient of about 5/3:1, and the second portion ofalternating layers have a layer thickness gradient of about 1.5:1.

In an alternate embodiment, the infrared reflecting film of the presentinvention may comprise a first portion of alternating layers comprisinga six layer alternating layer repeating unit or a multicomponent opticaldesign that reflects infrared light of wavelengths between about1200-2000 nm while minimizing higher order reflections that contributeto visible color, and a second portion of alternating layers having anAB repeat unit and substantially equal optical thicknesses which reflectinfrared light of wavelengths between about 700-1200 nm. Such acombination of alternating layers results in reflection of light acrossthe infrared wavelength region through about 2000 nm, and is commonlyknown as a “hybrid design”. This hybrid design may be provided asdescribed, for example, in U.S. Pat. No. 5,360,659, but has broaderapplication in that it is useful with any of the multicomponent opticaldesigns described herein. The layer thicknesses of both portions ofalternating layers can then be adjusted to place the reflecting bandwithin the infrared spectrum so as to minimize any perceived colorchange with angle.

Another useful film design is described in U.S. Ser. No. 09/066118entitled “Multicomponent Optical Body” filed on even date under AttorneyDocket No. 53543USA1A, which is incorporated herein by reference.Optical films and other optical bodies are described which exhibit afirst order reflection band for at least one polarization ofelectromagnetic radiation in a first region of the spectrum whilesuppressing at least the second, and preferably also at least the third,higher order harmonics of the first reflection band, while the %reflection of the first order harmonic remains essentially constant, orincreases, as a function of angle of incidence. This is accomplished byforming at least a portion of the optical body out of polymericmaterials A, B, and C which are arranged in a repeating sequence ABC,wherein A has refractive indices n_(x) ^(A), n_(y) ^(A), and n_(z) ^(A)along mutually orthogonal axes x, y, and z, respectively, B hasrefractive indices n_(x) ^(B), n_(y) ^(B), and n_(z) ^(B) along axes x,y and z, respectively, and C has refractive indices n_(x) ^(C), n_(y)^(C) and n_(z) ^(C) along axes x, y, and z, respectively, where axis zis orthogonal to the plane of the film or optical body, wherein n_(x)^(A)>n_(x) ^(B)>n_(x) ^(C) or n_(y) ^(A)>n_(y) ^(B)>n_(y) ^(C), andwherein n_(z) ^(C)≧n_(z) ^(B)≧n_(z) ^(A). Preferably, at least one ofthe differences n_(z) ^(A)−n_(z) and n_(z) ^(B)−n_(z) ^(C) is less thanor equal to about −0.05.

By designing the film or optical body within these constraints, at leastsome combination of second, third and forth higher-order reflections canbe suppressed without a substantial decrease of the first harmonicreflection with angle of incidence, particularly when the firstreflection band is in the infrared region of the spectrum. Such filmsand optical bodies are particularly useful as IR mirrors, and may beused advantageously as window films and in similar applications where IRprotection is desired but good transparency and low color are important.

Materials Selection and Processing

While the optical film of the present invention can be made withdielectric inorganic thin film stacks of materials such as indium tinoxide (ITO), silicon dioxide (SiO2), zirconium dioxide (ZrO2), ortitanium dioxide (TiO2) as described, for example, in EP 0 080 182 andU.S. Pat. Nos. 4,705,356 and 5,179,468, the preferred optical film is apolymeric multilayer film having alternating layers of polymericmaterials having high and low indices of refraction. The construction,materials, and optical properties of conventional multilayer polymericfilms are generally known, and were first described in Alfrey et al.,Polymer Engineering and Science, Vol. 9, No. 6, pp 400-404, November1969; Radford et al., Polymer Engineering and Science, Vol. 13, No. 3,pp 216-221, May 1973; and U.S. Pat. No. 3,610,729 (Rogers). Morerecently patents and publications including PCT InternationalPublication Number WO 95/17303 (Ouderkirk et al.), PCT InternationalPublication Number WO 96/19347 (Jonza et al.), U.S. Pat. No. 5,095,210(Wheatley et al.), and U.S. Pat. No. 5,149,578 (Wheatley et al.),discuss useful optical effects which can be achieved with large numbersof alternating thin layers of different polymeric materials that exhibitdiffering optical properties, in particular different refractive indicesin different directions. The contents of all of these references areincorporated by reference herein.

Multilayer polymeric films can include hundreds or thousands of thinlayers, and may contain as many materials as there are layers in thestack. For ease of manufacturing, preferred multilayer films have only afew different materials, and for simplicity those discussed hereintypically include only two. The multilayer film includes alternatinglayers of a first polymeric material having a first index of refraction,and a second polymeric material having a second index of refraction thatis different from that of the first material. The individual layers aretypically on the order of 0.05 micrometers to 0.45 micrometers thick. Asan example, the PCT Publication to Ouderkirk et al. discloses amultilayered polymeric film having alternating layers of crystallinenaphthalene dicarboxylic acid polyester and another selected polymer,such as copolyester or copolycarbonate, wherein the layers have athickness of less than 0.5 micrometers, and wherein the refractiveindices of one of the polymers can be as high as 1.9 in one directionand 1.64 in the other direction, thereby providing a birefringent effectwhich is usefull in the polarization of light.

Adjacent pairs of layers (one having a high index of refraction, and theother a low index) preferably have a total optical thickness that is ½of the wavelength of the light desired to be reflected, as shown in FIG.2. For maximum reflectivity the individual layers of a multilayerpolymeric film have an optical thickness that is ¼ of the wavelength ofthe light desired to be reflected, although other ratios of the opticalthicknesses within the layer pairs may be chosen for other reasons.These preferred conditions are expressed in Equations 1 and 2,respectively. Note that optical thickness is defined as the refractiveindex of a material multiplied by the actual thickness of the material,and that unless stated otherwise, all actual thicknesses discussedherein are measured after any orientation or other processing. Forbiaxially oriented multilayer optical stacks at normal incidence, thefollowing equation applies:

λ/2=t₁+t₂=n₁d₁+n₂d₂  Equation 1

λ/4=t₁=t₂=n₁d₁=n₂d₂  Equation 2

where λ=wavelength of maximum light reflection

t₁=optical thickness of the first layer of material

t₂=optical thickness of the second layer of material and

n₁=in-plane refractive index of the first material

n₂=in-plane refractive index of the second material

d₁=actual thickness of the first material

d₂=actual thickness of the second material

By creating a multilayer film with layers having different opticalthicknesses (for example, in a film having a layer thickness gradient),the film will reflect light of different wavelengths. An importantfeature of the present invention is the selection of layers havingdesired optical thicknesses (by selecting the actual layer thicknessesand materials) sufficient to reflect light in the near infrared portionof the spectrum. Moreover, because pairs of layers will reflect apredictable band width of light, as described below, individual layerpairs may be designed and made to reflect a given band width of light.Thus, if a large number of properly selected layer pairs are combined,superior reflectance of a desired portion of the near infrared spectrumcan be achieved.

The bandwidth of light desired to be blocked, i.e., not transmitted, ata zero degree observation angle in accordance with an optical body ofthe present invention is from approximately 700 to 1200 nm. Thus, thelayer pairs preferably have optical thicknesses ranging from 350 to 600nm (½ the wavelength of the light desired to be reflected) in order toreflect the near infrared light. More preferably, the multilayer filmwould have individual layers each having an optical thickness rangingfrom 175 to 300 nm (¼ the wavelength of the light desired to bereflected), in order to reflect the near infrared light. Assuming forpurposes of illustration that the first layer material has a refractiveindex of 1.66 (as does biaxially oriented PET), and the second layermaterial has a refractive index of 1.52 (as does the biaxially orientedthermoplastic polyester commercially available from Eastman ChemicalCo., Knoxville, Tenn., under the trade designation “Ecdel”), andassuming that both layers have the same optical thickness (¼wavelength), then the actual thicknesses of the first material layerswould range from approximately 105 to 180 nm, and the actual thicknessesof the second layers would range from approximately 115 to 197 nm. Theoptical properties of multilayer films such as this are discussed indetail below.

The various layers in the film preferably have different thicknesses.This is commonly referred to as the layer thickness gradient. A layerthickness gradient is selected to achieve the desired band width ofreflection. One common layer thickness gradient is a linear one, inwhich the thickness of the thickest layer pairs is a certain percentthicker than the thickness of the thinnest layer pairs. For example, a1.055:1 layer thickness gradient means that the thickest layer pair(adjacent to one major surface) is 5.5% thicker than the thinnest layerpair (adjacent to the opposite surface of the film). In anotherembodiment, the layer thickness could decrease, then increase, thendecrease again from one major surface of the film to the other. This isbelieved to provide sharper bandedges, and thus a sharper or more abrupttransition from clear to colored in the case of the present invention.This preferred method for achieving sharpened bandedges is describedmore fully in U.S. Ser. No. 09/006085 entitled “Optical Film withSharpened Bandedge” filed on even date under Attorney Docket No.53545USA7A.

Many different materials may be used in the dielectric color shiftingfilms of the present invention, depending on the specific application towhich the film is directed. Such materials include inorganic materialssuch as SiO₂, TiO₂, ZrO₂, or ITO, or organic materials such as liquidcrystals, and polymeric materials, including monomers, copolymers,grafted polymers, and mixtures or blends thereof. The exact choice ofmaterials for a given application will be driven by the desired matchand mismatch obtainable in the refractive indices between the variousoptical layers along a particular axis, as well as the desired physicalproperties in the resulting product.

Suitable polymeric materials for use in the optical films of the presentinvention may be amorphous, semicrystalline, or crystalline polymericmaterials. The films consist of at least two distinguishable polymershaving different indices of refraction. The number is not limited, andthree or more materials may be advantageously used in applicationswherein it is desirable to eliminate higher order harmonics that wouldotherwise reflect light in the visible region of the spectrum and give afilm with a colored appearance. For simplicity, the films will bedescribed further considering an optical stack made from only twomaterials.

A variety of polymer materials suitable for use in the present inventionhave been taught for use in making coextruded multilayer optical films.For example, in U.S. Pat. Nos. 4,937,134, 5,103,337, 5,1225,448,404,5,540,978, and 5,568,316 to Schrenk et al., and in 5,122,905, 5,122,906,and 5,126,880 to Wheatley and Schrenk. Of special interest arebirefringent polymers such as those described in 5,486,949 and 5,612,820to Schrenk et al, U.S. Application No. 08/402,041 to Jonza et al, andU.S. Ser. No. 09/006601 entitled “Modified Copolyesters and ImprovedMultilayer Reflective Films” filed on even date under Attorney DocketNo. 53550USA6A, all of which are herein incorporated by reference.Regarding the preferred materials from which the films are to be made,there are several conditions which should be met to make the multilayeroptical films of this invention. First, these films should consist of atleast two distinguishable polymers; the number is not limited, and threeor more polymers may be advantageously used in particular films. Second,at least one of the two required polymers, referred to as the “firstpolymer”, should have a stress optical coefficient having a largeabsolute value. In other words, it must be capable of developing a largebirefringence when stretched. Depending on the application, thebirefringence may be developed between two orthogonal directions in theplane of the film, between one or more in-plane directions and thedirection perpendicular to the film plane, or a combination of these.Third, the first polymer should be capable of maintaining birefringenceafter stretching, so that the desired optical properties are imparted tothe finished film. Fourth, the other required polymer, referred to asthe “second polymer”, should be chosen so that in the finished film, itsrefractive index, in at least one direction, differs significantly fromthe index of refraction of the first polymer in the same direction.Because polymeric materials are typically dispersive, that is, therefractive indices vary with wavelength, these conditions must beconsidered in terms of a particular spectral bandwidth of interest.

Other aspects of polymer selection depend on specific applications. Forpolarizing films, it is advantageous for the difference in the index ofrefraction of the first and second polymers in one film-plane directionto differ significantly in the finished film, while the difference inthe orthogonal film-plane index is minimized. If the first polymer has alarge refractive index when isotropic, and is positively birefringent(that is, its refractive index increases in the direction ofstretching), the second polymer will be chosen to have a matchingrefractive index, after processing, in the planar direction orthogonalto the stretching direction, and a refractive index in the direction ofstretching which is as low as possible. Conversely, if the first polymerhas a small refractive index when isotropic, and is negativelybirefringent, the second polymer will be chosen to have a matchingrefractive index, after processing, in the planar direction orthogonalto the stretching direction, and a refractive index in the direction ofstretching which is as high as possible.

Alternatively, it is possible to select a first polymer which ispositively birefringent and has an intermediate or low refractive indexwhen isotropic, or one which is negatively birefringent and has anintermediate or high refractive index when isotropic. In these cases,the second polymer may be chosen so that, after processing, itsrefractive index will match that of the first polymer in either thestretching direction or the planar direction orthogonal to stretching.Further, the second polymer will be chosen such that the difference inindex of refraction in the remaining planar direction is maximized,regardless of whether this is best accomplished by a very low or veryhigh index of refraction in that direction.

One means of achieving this combination of planar index matching in onedirection and mismatching in the orthogonal direction is to select afirst polymer which develops significant birefringence when stretched,and a second polymer which develops little or no birefringence whenstretched, and to stretch the resulting film in only one planardirection. Alternatively, the second polymer may be selected from amongthose which develop birefringence in the sense opposite to that of thefirst polymer (negative—positive or positive—negative). Anotheralternative method is to select both first and second polymers which arecapable of developing birefringence when stretched, but to stretch intwo orthogonal planar directions, selecting process conditions, such astemperatures, stretch rates, post-stretch relaxation, and the like,which result in development of unequal levels of orientation in the twostretching directions for the first polymer, and levels of orientationfor the second polymer such that one in-plane index is approximatelymatched to that of the first polymer, and the orthogonal in-plane indexis significantly mismatched to that of the first polymer. For example,conditions may be chosen such that the first polymer has a biaxiallyoriented character in the finished film, while the second polymer has apredominantly uniaxially oriented character in the finished film.

The foregoing is meant to be exemplary, and it will be understood thatcombinations of these and other techniques may be employed to achievethe polarizing film goal of index mismatch in one in-plane direction andrelative index matching in the orthogonal planar direction.

Different considerations apply to a reflective, or mirror, film.Provided that the film is not meant to have some polarizing propertiesas well, refractive index criteria apply equally to any direction in thefilm plane, so it is typical for the indices for any given layer inorthogonal in-plane directions to be equal or nearly so. It isadvantageous, however, for the film-plane indices of the first polymerto differ as greatly as possible from the film-plane indices of thesecond polymer. For this reason, if the first polymer has a high indexof refraction when isotropic, it is advantageous that it also bepositively birefringent. Likewise, if the first polymer has a low indexof refraction when isotropic, it is advantageous that it also benegatively birefringent. The second polymer advantageously developslittle or no birefringence when stretched, or develops birefringence ofthe opposite sense (positive—negative or negative—positive), such thatits film-plane refractive indices differ as much as possible from thoseof the first polymer in the finished film. These criteria may becombined appropriately with those listed above for polarizing films if amirror film is meant to have some degree of polarizing properties aswell.

Colored films can be regarded as special cases of mirror and polarizingfilms. Thus, the same criteria outlined above apply. The perceived coloris a result of reflection or polarization over one or more specificbandwidths of the spectrum. The bandwidths over which a multilayer filmof the current invention is effective will be determined primarily bythe distribution of layer thicknesses employed in the optical stack(s),but consideration must also be given to the wavelength dependence, ordispersion, of the refractive indices of the first and second polymers.It will be understood that the same rules apply to the infrared andultraviolet wavelengths as to the visible colors.

Absorbance is another consideration. For most applications, it isadvantageous for neither the first polymer nor the second polymer tohave any absorbance bands within the bandwidth of interest for the filmin question. Thus, all incident light within the bandwidth is eitherreflected or transmitted. However, for some applications, it may beuseful for one or both of the first and second polymer to absorbspecific wavelengths, either totally or in part.

Polyethylene 2,6-naphthalate (PEN) is frequently chosen as a firstpolymer for films of the present invention. It has a large positivestress optical coefficient, retains birefringence effectively afterstretching, and has little or no absorbance within the visible range. Italso has a large index of refraction in the isotropic state. Itsrefractive index for polarized incident light of 550 nm wavelengthincreases when the plane of polarization is parallel to the stretchdirection from about 1.64 to as high as about 1.9. Its birefringence canbe increased by increasing its molecular orientation which, in turn, maybe increased by stretching to greater stretch ratios with otherstretching conditions held fixed.

Other semicrystalline naphthalene dicarboxylic polyesters are alsosuitable as first polymers. Polybutylene 2,6-Naphthalate (PBN) is anexample. These polymers may be homopolymers or copolymers, provided thatthe use of comonomers does not substantially impair the stress opticalcoefficient or retention of birefringence after stretching. The term“PEN” herein will be understood to include copolymers of PEN meetingthese restrictions. In practice, these restrictions imposes an upperlimit on the comonomer content, the exact value of which will vary withthe choice of comonomer(s) employed. Some compromise in these propertiesmay be accepted, however, if comonomer incorporation results inimprovement of other properties. Such properties include but are notlimited to improved interlayer adhesion, lower melting point (resultingin lower extrusion temperature), better rheological matching to otherpolymers in the film, and advantageous shifts in the process window forstretching due to change in the glass transition temperature.

Suitable comonomers for use in PEN, PBN or the like may be of the diolor dicarboxylic acid or ester type. Dicarboxylic acid comonomers includebut are not limited to terephthalic acid, isophthalic acid, phthalicacid, all isomeric naphthalenedicarboxylic acids (2,6-, 1,2-, 1,3-,1,4-, 1,5-, 1,6-, 1,7-, 1,8-, 2,3-, 2,4-, 2,5-, 2,7-, and 2,8-),bibenzoic acids such as 4,4′-biphenyl dicarboxylic acid and its isomers,trans4,4′-stilbene dicarboxylic acid and its isomers, 4,4′-diphenylether dicarboxylic acid and its isomers, 4,4′-diphenylsulfonedicarboxylic acid and its isomers, 4,4′-benzophenone dicarboxylic acidand its isomers, halogenated aromatic dicarboxylic acids such as2-chloroterephthalic acid and 2,5-dichloroterephthalic acid, othersubstituted aromatic dicarboxylic acids such as tertiary butylisophthalic acid and sodium sulfonated isophthalic acid, cycloalkanedicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid and itsisomers and 2,6-decahydronaphthaletic dicarboxylic acid and its isomers,bi- or multi-cyclic dicarboxylic acids (such as the various isomericnorbomane and norbornene dicarboxylic acids, adamantane dicarboxylicacids, and bicyclo-octane dicarboxylic acids), alkane dicarboxylic acids(such as sebacic acid, adipic acid, oxalic acid, malonic acid, succinicacid, glutaric acid, azelaic acid, and dodecane dicarboxylic acid.), andany of the isomeric dicarboxylic acids of the fused-ring aromatichydrocarbons (such as indene, anthracene, pheneanthrene, benzonaphthene,fluorene and the like). Alternatively, alkyl esters of these monomers,such as dimethyl terephthalate, may be used.

Suitable diol comonomers include but are not limited to linear orbranched alkane diols or glycols (such as ethylene glycol, propanediolssuch as trimethylene glycol, butanediols such as tetramethylene glycol,pentanediols such as neopentyl glycol, hexanediols,2,2,4-trimethyl-1,3-pentanediol and higher diols), ether glycols (suchas diethylene glycol, triethylene glycol, and polyethylene glycol),chain-ester diols such as3-hydroxy-2,2-dimethylpropyl-3-hydroxy-2,2-dimethyl propanoate,cycloalkane glycols such as 1,4-cyclohexanedimethanol and its isomersand 1,4-cyclohexanediol and its isomers, bi- or multicyclic diols (suchas the various isomeric tricyclodecane dimethanols, norbornanedimethanols, norbomene dimethanols, and bicyclo-octane dimethanols),aromatic glycols (such as 1,4-benzenedimethanol and its isomers,1,4-benzenediol and its isomers, bisphenols such as bisphenol A,2,2′-dihydroxy biphenyl and its isomers, 4,4′-dihydroxymethyl biphenyland its isomers, and 1,3-bis(2-hydroxyethoxy)benzene and its isomers),and lower alkyl ethers or diethers of these diols, such as dimethyl ordiethyl diols.

Tri- or polyfunctional comonomers, which can serve to impart a branchedstructure to the polyester molecules, can also be used. They may be ofeither the carboxylic acid, ester, hydroxy or ether types. Examplesinclude, but are not limited to, trimellitic acid and its esters,trimethylol propane, and pentaerythritol.

Also suitable as comonomers are monomers of mixed functionality,including hydroxycarboxylic acids such as parahydroxybenzoic acid and6-hydroxy-2-naphthalenecarboxylic acid, and their isomers, and tri- orpolyfunctional comonomers of mixed functionality such as5-hydroxyisophthalic acid and the like.

Polyethylene terephthalate (PET) is another material that exhibits asignificant positive stress optical coefficient, retains birefringenceeffectively after stretching, and has little or no absorbance within thevisible range. Thus, it and its high PET-content copolymers employingcomonomers listed above may also be used as first polymers in someapplications of the current invention.

When a naphthalene dicarboxylic polyester such as PEN or PBN is chosenas first polymer, there are several approaches which may be taken to theselection of a second polymer. One preferred approach for someapplications is to select a naphthalene dicarboxylic copolyester (coPEN)formulated so as to develop significantly less or no birefringence whenstretched. This can be accomplished by choosing comonomers and theirconcentrations in the copolymer such that crystallizability of the coPENis eliminated or greatly reduced. One typical formulation employs as thedicarboxylic acid or ester components dimethyl naphthalate at from about20 mole percent to about 80 mole percent and dimethyl terephthalate ordimethyl isophthalate at from about 20 mole percent to about 80 molepercent, and employs ethylene glycol as diol component. Of course, thecorresponding dicarboxylic acids may be used instead of the esters. Thenumber of comonomers which can be employed in the formulation of a coPENsecond polymer is not limited. Suitable comonomers for a coPEN secondpolymer include but are not limited to all of the comonomers listedabove as suitable PEN comonomers, including the acid, ester, hydroxy,ether, tri- or polyfunctional, and mixed functionality types.

Often it is useful to predict the isotropic refractive index of a coPENsecond polymer. A volume average of the refractive indices of themonomers to be employed has been found to be a suitable guide. Similartechniques well-known in the art can be used to estimate glasstransition temperatures for coPEN second polymers from the glasstransitions of the homopolymers of the monomers to be employed.

In addition, polycarbonates having a glass transition temperaturecompatible with that of PEN and having a refractive index similar to theisotropic refractive index of PEN are also useful as second polymers.Polyesters, copolyesters, polycarbonates, and copolycarbonates may alsobe fed together to an extruder and transesterified into new suitablecopolymeric second polymers.

It is not required that the second polymer be a copolyester orcopolycarbonate. Vinyl polymers and copolymers made from monomers suchas vinyl naphthalenes, styrenes, ethylene, maleic anhydride, acrylajes,acetates, and methacrylates may be employed. Condensation polymers otherthan polyesters and polycarbonates may also be used. Examples include:polysulfones, polyamides, polyurethanes, polyamic acids, and polyimides.Naphthalene groups and halogens such as chlorine, bromine and iodine areuseful for increasing the refractive index of the second polymer to adesired level. Acrylate groups and fluorine are particularly useful indecreasing refractive index when this is desired.

It will be understood from the foregoing discussion that the choice of asecond polymer is dependent not only on the intended application of themultilayer optical film in question, but also on the choice made for thefirst polymer, and the processing conditions employed in stretching.Suitable second polymer materials include but are not limited topolyethylene naphthalate (PEN) and isomers thereof (such as 2,6-, 1,4-,1,5-, 2,7-, and 2,3-PEN), polyalkylene terephthalates (such aspolyethylene terephthalate, polybutylene terephthalate, andpoly-1,4-cyclohexanedimethylene terephthalate), other polyesters,polycarbonates, polyarylates, polyamides (such as nylon 6, nylon 11,nylon 12, nylon 4/6, nylon 6/6, nylon 6/9, nylon 6/10, nylon 6/12, andnylon 6/T), polyimides (including thermoplastic polyimides andpolyacrylic imides), polyamide-imides, polyether-amides,polyetherimides, polyaryl ethers (such as polyphenylene ether and thering-substituted polyphenylene oxides), polyarylether ketones such aspolyetheretherketone (“PEEK”), aliphatic polyketones (such as copolymersand terpolymers of ethylene and/or propylene with carbon dioxide),polyphenylene sulfide, polysulfones (including polyethersulfones andpolyaryl sulfones), atactic polystyrene, syndiotactic polystyrene(“sPS”) and its derivatives (such as syndiotactic poly-alpha-methylstyrene and syndiotactic polydichlorostyrene), blends of any of thesepolystyrenes (with each other or with other polymers, such aspolyphenylene oxides), copolymers of any of these polystyrenes (such asstyrene-butadiene copolymers, styrene-acrylonitrile copolymers, andacrylonitrile-butadiene-styrene terpolymers), polyacrylates (such aspolymethyl acrylate, polyethyl acrylate, and polybutyl acrylate),polymethacrylates (such as polymethyl methacrylate, polyethylmethacrylate, polypropyl methacrylate, and polyisobutyl methacrylate),cellulose derivatives (such as ethyl cellulose, cellulose acetate,cellulose propionate, cellulose acetate butyrate, and cellulosenitrate), polyalkylene polymers (such as polyethylene, polypropylene,polybutylene, polyisobutylene, and poly(4-methyl)pentene), fluorinatedpolymers and copolymers (such as polytetrafluoroethylene,polytrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride,fluorinated ethylene-propylene copolymers, perfluoroalkoxy resins,polychlorotrifluoroethylene, polyethylene-co-trifluoroethylene,polyethylene-co-chlorotrifluoroethylene), chlorinated polymers (such aspolyvinylidene; chloride and polyvinyl chloride), polyacrylonitrile,polyvinylacetate, polyethers (such as polyoxymethylene and polyethyleneoxide), ionomeric resins, elastomers (such as polybutadiene,polyisoprene, and neoprene), silicone resins, epoxy resins, andpolyurethanes.

Also suitable are copolymers, such as the copolymers of PEN discussedabove as well as any other non- naphthalene group-containingcopolyesters which may be formulated from the above lists of suitablepolyester comonomers for PEN. In some applications, especially when PETserves as the first polymer, copolyesters based on PET and comonomersfrom said lists above (coPETs) are especially suitable. In addition,either first or second polymers may consist of miscible or immiscibleblends of two or more of the above-described polymers or copolymers(such as blends of sPS and atactic polystyrene, or of PEN and sPS). ThecoPENs and coPETs described may be synthesized directly, or may beformulated as a blend of pellets where at least one component is apolymer based on naphthalene dicarboxylic acid or terephthalic acid andother components are polycarbonates or other polyesters, such as a PET,a PEN, a coPET, or a co-PEN.

Another preferred family of materials for the second polymer for someapplications are the syndiotactic vinyl aromatic polymers, such assyndiotactic polystyrene. Syndiotactic vinyl aromatic polymers useful inthe current invention include poly(styrene), poly(alkyl styrene)s, poly(aryl styrene)s, poly(styrene halide)s, poly(alkoxy styrene)s,poly(vinyl ester benzoate), poly(vinyl naphthalene), poly(vinylstyrene),and poly(acenaphthalene), as well as the hydrogenated polymers andmixtures or copolymers containing these structural units. Examples ofpoly(alkyl styrene)s include the isomers of the following: poly(methylstyrene), poly(ethyl styrene), poly(propyl styrene), and poly(butylstyrene). Examples of poly(aryl styrene)s include the isomers ofpoly(phenyl styrene). As for the poly(styrene halide)s, examples includethe isomers of the following: poly(chlorostyrene), poly(bromostyrene),and poly(fluorostyrene). Examples of poly(alkoxy styrene)s include theisomers of the following: poly(methoxy styrene) and poly(ethoxystyrene). Among these examples, particularly preferable styrene grouppolymers, are: polystyrene, poly(p-methyl styrene), poly(m-methylstyrene), poly(p-tertiary butyl styrene), poly(p-chlorostyrene),poly(m-chloro styrene), poly(p-fluoro styrene), and copolymers ofstyrene and p-methyl styrene.

Furthermore, comonomers may be used to make syndiotactic vinyl aromaticgroup copolymers. In addition to the monomers for the homopolymerslisted above in defining the syndiotactic vinyl aromatic polymers group,suitable comonomers include olefin monomers (such as ethylene,propylene, butenes, pentenes, hexenes, octenes or decenes), dienemonomers (such as butadiene and isoprene), and polar vinyl monomers(such as cyclic diene monomers, methyl methacrylate, maleic acidanhydride, or acrylonitrile).

The syndiotactic vinyl aromatic copolymers of the present invention maybe block copolymers, random copolymers, or alternating copolymers.

The syndiotactic vinyl aromatic polymers and copolymers referred to inthis invention generally have syndiotacticity of higher than 75% ormore, as determined by carbon-13 nuclear magnetic resonance. Preferably,the degree of syndiotacticity is higher than 85% racemic diad, or higherthan 30%, or more preferably, higher than 50%, racemic pentad.

In addition, although there are no particular restrictions regarding themolecular weight of these syndiotactic vinyl aromatic polymers andcopolymers, preferably, the weight average molecular weight is greaterthan 10,000 and less than 1,000,000, and more preferably, greater than50,000 and less than 800,000.

The syndiotactic vinyl aromatic polymers and copolymers may also be usedin the form of polymer blends with, for instance, vinyl aromatic grouppolymers with atactic structures, vinyl aromatic group polymers withisotactic structures, and any other polymers that are miscible with thevinyl aromatic polymers. For example, polyphenylene ethers show goodmiscibility with many of the previous described vinyl aromatic grouppolymers.

When a polarizing film is made using a process with predominantlyuniaxial stretching, particularly preferred combinations of polymers foroptical layers include PEN/coPEN, PET/coPET, PEN/sPS, PET/sPS,PEN/Eastar,™ and PET/Eastar,™ where “coPEN” refers to a copolymer orblend based upon naphthalene dicarboxylic acid (as described above) andEastar™ is a polyester or copolyester (believed to comprisecyclohexanedimethylene diol units and terephthalate units) commerciallyavailable from Eastman Chemical Co. When a polarizing film is to be madeby manipulating the process conditions of a biaxial stretching process,particularly preferred combinations of polymers for optical layersinclude PEN/coPEN, PEN/PET, PEN/PBT, PEN/PETG and PEN/PETcoPBT, where“PBT” refers to polybutylene terephthalate, “PETG” refers to a copolymerof PET employing a second glycol (usually cyclohexanedimethanol), and“PETcoPBT” refers to a copolyester of terephthalic acid or an esterthereof with a mixture of ethylene glycol and 1,4-butanediol.

Particularly preferred combinations of polymers for optical layers inthe case of mirrors or colored films include PEN/PMMA, PET/PMMA,PEN/“Ecdel”, PETP“Ecdel”, PEN/sPS, PET/sPS, PEN/coPET, PEN/PETG, andPEN/a fluoropolymer commercially available from Minnesota Mining andManufacturing Company (3M), St. Paul, Minn. under the trade designation“THV”, where “PMMA” refers to polymethyl methacrylate, “coPET” refers toa copolymer or blend based upon terephthalic acid (as described above).and “PETG” refers to a copolymer of PET employing a second glycol(usually cyclohexanedimethanol).

For mirror films, a match of the refractive indices of the first polymerand second polymer in the direction normal to the film plane ispreferred, because it provides for constant reflectance with respect tothe angle of incident light (that is, there is no Brewster's angle). Forexample, at a specific wavelength, the in-plane refractive indices mightbe 1.76 for biaxially oriented PEN, while the film plane-normalrefractive index might fall to 1.49. When PMMA is used as the secondpolymer in the multilayer construction, its refractive index at the samewavelength, in all three directions, might be 1.495. Another example isthe PET/“Ecdel” system, in which the analogous indices might be 1.66 and1.51 for PET, while the isotropic index of“Ecdel” might be 1.52. Thecrucial property is that the normal-to-plane index for one material mustbe closer to the in-plane indices of the other material than to its ownin-plane indices. As described previously, it is sometimes preferred forthe multilayer optical films of the current invention to consist of morethan two distinguishable polymers. A third or subsequent polymer mightbe fruitfully employed as an adhesion-romoting layer between the firstpolymer and the second polymer within an optical stack, as an additionalcomponent in a stack for optical purposes, as a protective boundarylayer between optical stacks, as a skin layer, as a functional coating,or for any other purpose. As such, the composition of a third orsubsequent polymer, if any, is not limited. Preferred multicomponentfilms include those described in U.S. Ser. No. 09/066118 filed on evendate under Attorney Docket No. 53543USA1A entitled “MulticomponentOptical Body”, hereby incorporated by reference.

Optical Properties

The reflectance characteristics of multilayer films are determined byseveral factors, the most important of which for purposes of thisdiscussion are the indices of refraction for each layer of the filmstack. In particular, reflectivity depends upon the relationship betweenthe indices of refraction of each material in the x, y, and z directions(n_(x), n_(y), n_(z),). Different relationships between the threeindices lead to three general categories of materials: isotropic,uniaxially birefringent, and biaxially birefringent. The latter two areimportant to the optical performance of the present invention.

Uniaxially Birefringent Materials (Mirrors)

In a uniaxially birefringent material, two indices (typically along thex and y axes, or n_(x), and n_(y)) are equal, and different from thethird index (typically along the z axis, or n_(z)). The x and y axes aredefined as the in-plane axes, in that they represent the plane of agiven layer within the multilayer film, and the respective indices n.and n_(y) are referred to as the in-plane indices.

One method of creating a uniaxially birefringent system is to biaxiallyorient (stretch along two axes) a multilayer polymeric film. Biaxialorientation of the multilayer film results in differences betweenrefractive indices of adjoining layers for planes parallel to both axes,resulting in the reflection of light in both planes of polarization. Auniaxially birefringent material can have either positive or negativeuniaxial birefringence. Positive uniaxial birefringence occurs when theindex of refraction in the z direction (n_(z)) is greater than thein-plane indices (n_(x) and n_(y)). Negative uniaxial birefringenceoccurs when the index of refraction in the z direction (n_(z)) is lessthan the in-plane indices (n_(x) and n_(y)). It can be shown that whenn_(1z) is selected to match n_(2x)=n_(2y)=n_(2z) and the multilayer filmis biaxially oriented, there is no Brewster's angle for p-polarizedlight and thus there is constant reflectivity for all angles ofincidence. In other words, properly designed multilayer films that areoriented in two mutually perpendicular in-plane axes reflect anextraordinarily high percentage of incident light, and are highlyefficient mirrors. By selecting the layers as previously described toreflect near infrared light, and positioning the reflective bandedgewithin the infrared region such that even at grazing angles of incidencethe reflectance band does not shift into the visible region of thespectrum, an infrared mirror can be made that is transparent in thevisible region of the spectrum, even at high angles of incidence.Alternatively, if some color is acceptable or desirable, the reflectivebandedge may be positioned so that even at grazing angles of incidence,the reflectance band does not shift past the colored portion of thevisible spectrum, thereby maintaining the same visible color at grazingangles as at normal angle of incidence. This same effect may be achievedby positioning two uniaxially oriented films (discussed below) withtheir respective orientation axes at 90° to each other.

Biaxially Birefringent Materials (Polarizers)

In a biaxially birefringent material, all three indices are different.Biaxially birefringent materials are important to the film of thepresent invention. A biaxially birefringent system can be made byuniaxially orienting (stretching along one axis) the multilayerpolymeric film, such as along the x direction in FIG. 2. A biaxiallybirefringent multilayer film can be designed to provide highreflectivity for light with its plane of polarization parallel to oneaxis, for all angles of incidence, and simultaneously have lowreflectivity (high transmissivity) for light with its plane ofpolarization parallel to the other axis at all angles of incidence. As aresult, the biaxially birefringent system acts as a polarizer,reflecting light of one polarization and transmitting light of the otherpolarization. Stated differently, a polarizing film is one that receivesincident light of random polarity (light vibrating in planes at randomangles), and allows incident light rays of one polarity (vibrating inone plane) to pass through the film, while reflecting incident lightrays of the other polarity (vibrating in a plane perpendicular to thefirst plane). By controlling the three indices of refraction—n_(x),n_(y), and n_(z)—the desired polarizing effects can be obtained. If thelayers were appropriately designed to reflect light in the nearinfrared, and the reflective band positioned within the infrared regionsuch that even at grazing angles of incidence the reflectance band doesnot shift into the visible region of the spectrum, an infrared polarizercan be made that is transparent in the visible region of the spectrum,even at high angles of incidence. Alternatively, if some color isacceptable or desirable, the reflective bandedge may be positioned sothat even at grazing angles of incidence, the reflectance band does notshift past the colored portion of the visible spectrum, therebymaintaining the same visible color at grazing angles as at normal angleof incidence. Two crossed sheets of biaxially birefringent film wouldyield a highly efficient mirror, and the films would perform similar toa single uniaxially birefringent film.

A novel way of making multilayer polymeric polarizers using biaxialorientation is described U.S. Ser. No. 09/066455 filed on even dateunder Attorney Docket No. 53546USA5A entitled “An Optical Film andProcess for Manufacture Thereof”, hereby incorporated by reference. Inthis approach, two polymers capable of permanent birefringence are drawnsequentially such that in the first draw, the conditions are chosen toproduce little birefringence in one of the materials, and considerablebirefringence in the other. In the second draw, the second materialdevelops considerable birefringence, sufficient to match the finalrefractive index of the first material in that direction. Often thefirst material assumes an in-plane biaxial character after the seconddraw. An example of a system that produces a good polarizer from biaxialorientation is PEN/PET. In that case, the indices of refraction can beadjusted over a range of values. The following set of valuesdemonstrates the principle: for PEN, n_(1x)=1.68, n_(1y)=1.82,n_(1z)=1.49; for PET n_(1x)=1.67, n_(1y)=1.56 and n1z=1.56, all at 632.8nm. Copolymers of PEN and PET may also be used. For example, a copolymercomprising approximately 10% PEN subunits and 90% PET subunits by weightmay replace the PET homopolymer in the construction. Indices for thecopolymer under similar processing are about n_(1x)=1.67, n_(1y)=1.62,n_(1z)=1.52, at 632.8 nm. There is a good match of refractive indices inthe x direction, a large difference (for strong reflection) in the ydirection, and a small difference in the z direction. This small z indexdifference minimizes unwanted color leaks at shallow observation angles.The film formed by biaxial orientation is strong in all planardirections, while uniaxially oriented polarizer is prone to splitting.Depending on the application, either approach has merit.

To make an infrared reflecting film with minimal or no visible perceivedcolor, the infrared reflecting multilayer film of the present inventionmay be designed so that the reflecting band is positioned within theinfrared region at such a wavelength that it does not reflect red lightat angles less than the angle of use. If the reflection band is notpositioned sufficiently far into the infrared, then the film willreflect red at angles greater than the angle of use. Because cyan is bydefinition the subtraction of red light from white light, the filmappears cyan in transmission. The amount of red light reflected, andthus the degree to which the film appears cyan, depends on theobservation angle and the reflected band width. As shown in FIG. 1, theobservation angle alpha is measured between the photoreceptor (typicallya human eye) and the observation axis perpendicular to the plane of thefilm. When the observation angle is approximately zero degrees, verylittle visible light of any color is reflected by the multilayer film,and the film appears clear against a diffuse white background (or blackagainst a black background). When the observation angle exceeds apredetermined shift angle and the short wavelength bandedge has not beenpositioned properly within the infrared, a substantial portion of thered light is reflected by the multilayer film, and the film appears cyanagainst a diffuse white background (or red against a black background).As the observation angle increases toward 90 degrees, more red light isreflected by the multilayer film, and the cyan appears to be evendeeper. For some applications, the shift into the red region of thespectrum may be acceptable, for example, if the film is already cyan inappearance at normal angles, for example, because of the incorporationof an absorbing dye, then the short wavelength bandedge may shift beyondthe visible into the red region so long as it does not move beyond theabsorption bandedge of the dye to cause a change in the perceived colorwith angle.

One common description of reflectance band width depends on therelationship between the in-plane indices of refraction of the materialsin the stack, as shown by the following equation:

Band width=(4λ/π)sin⁻¹[(1−(n₂/n₁))/(1+(n₂/n₁))]  Equation 3

Thus, if n₁ is close to n₂, the reflectance peak is very narrow. Forexample, in the case of a multilayer film having alternating layers ofPET (n₁=1.66) and Ecdel (n₂=1.52) of the same optical thickness,selected for λ=750 nm minimum transmission, the breadth or band width ofthe transmission minimum is about 42 nm. In the case of a multilayerfilm having alternating layers of PEN (n₁=1.75) and PMMA (n₂=1.49) underthe same conditions, the band width is 77 nm. To reflect as much of thesolar spectrum as possible without having higher order harmonics giveperceptible color, the reflective band of the film of the presentinvention should be designed to cover from about 850 nm to about 1200nm. The band width for a given pair of materials may be estimated fromEquation 3, multiplying by the layer thickness ratio. The center of thereflectance band is calculated from Equations 1 or 2 so that it ispositioned approximately one half band width from the desired locationof the lower bandedge.

The value of the blue shift with angle of incidence in any thin filmstack can be derived from the basic wavelength tuning formula for anindividual layer, shown as Equation 4, below:

λ/4=ΣndCos θ  Equation 4

where

λ=design wavelength (the bandedge will actually extend below λ)

θ=angle of incidence measured from perpendicular in that layer

n=index of refraction for the material layer for the given direction andpolarization of the light traveling through the layer, and

d=actual thickness of the layer.

In an isotropic thin film stack, only the value of (Cos θ) decreases asθ increases. In a birefringent film, however, both n and (Cos θ)decrease for p-polarized light as θ increases. When the unit cellincludes one or more layers of a negatively birefringent material suchas PEN, the p-polarized light senses the low z-index value instead ofonly the in-plane value of the index, resulting in a reduced effectiveindex of refraction for the negatively birefringent layers. Accordingly,the effective low z-index caused by the presence of negativelybirefringent layers in the unit cell creates a secondary blue shift inaddition to the blue shift present in an isotropic thin stack. Thecompounded effects result in a greater blue shift of the spectrumcompared to film stacks composed entirely of isotropic materials. Theactual blue shift will be determined by the thickness weighted averagechange in λ with angle of incidence for all material layers in the unitcell. Thus, the blue shift can be enhanced or lessened by adjusting therelative thickness of the birefringent layer(s) to the isotropiclayer(s) in the unit cell. This will result in f-ratio changes that mustfirst be considered in the product design. The maximum blue shift inmirrors is attained by using negatively uniaxially birefringentmaterials in all layers of the stack. The minimum blue shift is attainedby using only uniaxially positive birefringent materials in the opticalstack. For polarizers, biaxially birefringent materials are used, butfor the simple case of light incident along one of the major axes of abirefringent thin film polarizer, the analysis is the same for bothuniaxial and biaxial films. For directions between the major axes of apolarizer, the effect is still observable but the analysis is morecomplex.

For mirror films made with PEN with equal stretch ratios along the twomajor axes of the film, the in-plane/z-axis index differential of thePEN layers is about 1.75-1.50. This index differential is less forPET-based films (i.e., about 1.66-1.50). For polarizers, with lightincident with the plane of polarization along the extinction axis, theeffect is even more pronounced because the difference in the PENin-plane index compared to the PEN z-axis index is much greater (about1.85-1.50), resulting in an even greater blue shift for p-polarizedlight than that observed in isotropic multilayer film stacks. If onlyuniaxially positive birefringent materials were used in the stack, theblue shift would be diminished compared to isotropic optical films.

For the uniaxially birefringent case of PEN/PMMA, the angular dependenceof the red light reflectance is illustrated in FIGS. 3 and 4. In thosegraphs, the percent of transmitted light is plotted along the verticalaxis, and the wavelengths of light are plotted along the horizontalaxis. Note that because the percentage of light transmitted is simply 1minus the percentage of reflected light (absorption is negligible),information about light transmission also provides information aboutlight reflection. The spectra provided in FIGS. 3 and 4 are taken from acomputerized optical modeling system, and actual performance typicallycorresponds relatively closely with predicted performance. Surfacereflections contribute to a decreased transmission in both the computermodeled and measured spectra. In examples for which actual samples weretested, a spectrometer available from the Perkin Elmer Corporation ofNorwalk, Conn. under the designation Lambda 19 was used to measureoptical transmission of light at the angles indicated.

A uniaxially birefringent film having a total of 224 alternating layersof PEN (n_(x,y)=1.75; n_(z)=1.5) and PMMA (n_(x,y,z)=1.5) with a linearratio of thickest layer pairs to thinnest of 1.13:1 was modeled. Thespectra for this ideal film at a zero degree observation angle and a 60degree observation angle are plotted in FIGS. 3 and 4, respectively. Thelow wavelength bandedge for both the s- and p-polarized light shifttogether from about 750 nm to about 600 nm and transmission is minimizedin the desired range of the spectrum, so that to the eye, a very sharpcolor shift is achieved. In fact, the concurrent shift of the s- andp-polarized light is a desirable aspect of the present invention. InFIGS. 3 and 4, this effect may be observed by determining whether thelow wavelength bandedges of the s- and p-polarized light spectra arespaced apart or not.

For comparison, a 24 layer construction of zirconia and silica wasmodeled to demonstrate the shift observed for multilayer films made fromisotropic materials.

The refractive index of zirconia was n_(x,y,x)=1.93, the refractiveindex of silica was n_(x,y,z)=1.45, and the model assumed a linear layerthickness gradient in which the thickest layer pair was 1.12 timesthicker than the thinnest layer pair. At a zero degree observationangle, the two films spectra look similar (compare FIG. 5 to FIG. 3),and to the naked eye, both would be clear. As shown in FIG. 6, however,the low wavelength bandedge for p-polarized light viewed at a 60 degreeobservation angle shifts by about 100 nm, while that for s-polarizedlight shifts by about 150 nm. This construction does not exhibit anabrupt change from clear to cyan because the s- and p-polarized light donot shift together with change in angle.

Typically, the value for the shift for p-polarized light for aconstruction with an isotropic film would likely be between the shiftachieved using a birefringent film and that achieved using an inorganicisotropic film, depending on the index of refraction of the specificmaterials used.

It is believed that one way to design a multilayer film in which thosebandedges are coincident is to choose materials with an F ratio ofapproximately 0.25. The F ratio, usually used to describe the F ratio ofthe birefringent layer, is calculated as shown in Equation 5:

F ratio=n₁d₁/(n₁d₁+n₂d₂)

where n and d are the refractive index and the actual thickness of thelayers, respectively. When the F ratio of the birefringent layer isapproximately 0.75, there is a significant separation between the lowerbandedges of the s- and p-polarized light spectra, and when the F ratiois approximately 0.5, there remains a noticeable separation. At an Fratio of 0.25, however, the lower bandedges of the s- and p-polarizedlight spectra are virtually coincident, resulting in a film having asharp color transition. Stated in different terms, it is most desirableto have the lower bandedges of the s- and p-polarized light spectrawithin approximately 20 nm of each other, and more desirable to havethem within approximately 10 nm of each other, to obtain the desiredeffect. For the modeled cases of FIGS. 3-6, an F ratio of 0.5 has beenused.

The optical theory underlying the modeled data described above will nowbe described in greater detail. A dielectric reflector is composed oflayer groups that have two or more layers of alternating high and lowindex of refraction. Each group has a halfwave optical thickness thatdetermines the wavelength of the reflection band. Typically, many setsof halfwaves are used to build a stack that has reflective power over arange of wavelengths. Most stack designs have sharp reflectivitydecreases at higher and lower wavelengths, know as bandedges. The edgeabove the halfwave position is the high wavelength bandedge, l_(BEhi),and the one below is the low wavelength bandedge, l_(BElo). These areillustrated in FIG. 8. The center, edges, and width of a reflection bandchange with incident angle.

The reflecting band can be exactly calculated by using a characteristicmatrix method. The characteristic matrix relates the electric field atone interface to that at the next. It has terms for each interface andeach layer thickness. By using effective indicies for interface andphase terms, both anisotropic and isotropic materials can be evaluated.The characteristic matrix for the halfwave is the product of the matrixfor each layer of the halfwave. The characteristic matrix for each layeris given by Equation 6: $\begin{matrix}{M_{i} = {\begin{bmatrix}M_{11} & M_{12} \\M_{21} & M_{22}\end{bmatrix} = \begin{bmatrix}\frac{\exp \left\lbrack \beta_{i} \right\rbrack}{t_{i}} & \frac{r_{i}{\exp \left\lbrack {- \beta_{i}} \right\rbrack}}{t_{i}} \\\frac{r_{i}{\exp \left\lbrack {- \beta_{i}} \right\rbrack}}{t_{i}} & \frac{\exp \left\lbrack \beta_{i} \right\rbrack}{t_{i}}\end{bmatrix}}} & \text{Equation~~6}\end{matrix}$

where r_(i) and t_(i) are the Fresnel coefficients for the interfacereflection of the i^(th) interface, and b_(i) is the phase thickness ofthe i^(th) layer.

The characteristic matrix of the entire stack is the product of thematrix for each layer. Other useful results, such as the totaltransmission and reflection of the stack, can be derived from thecharacteristic matrix. The Fresnel coefficients for the i^(th) interfaceare given by Equations 7(a) and 7(b): $\begin{matrix}{r_{i} = {{\frac{n_{i} - n_{i - 1}}{n_{i} + n_{i - 1}}\quad {and}\quad t_{i}} = \frac{2n_{i}}{n_{i} + n_{i - 1}}}} & \text{Equations~~7(a); 7(b)}\end{matrix}$

The effective indicies used for the Fresnel coefficients are given byEquations 8(a) and 8(b): $\begin{matrix}{n_{is} = {\frac{\sqrt{n_{ix}^{2} - {n_{o}^{2}\sin^{2}\theta_{o}}}}{\cos \quad \theta_{o}}\quad {\text{(for}\text{s}\text{polarized light)}}}} & \text{Equation~~8(a)}\end{matrix}$

$\begin{matrix}{n_{ip} = {\frac{n_{ix}n_{iz}\cos \quad \theta_{o}}{\sqrt{n_{iz}^{2} - {n_{o}^{2}\sin^{2}\theta_{o}}}}\quad {\text{(for}\text{p}\text{polarized light)}}}} & \text{Equation~~8(b)}\end{matrix}$

When these indicies are used, then the Fresnel coefficients areevaluated at normal incidence. The incident material has an index ofn_(o) and an angle of q_(o).

The total phase change of a halfwave pair, one or both may haveanisotropic indicies. Analytical expressions for the effectiverefractive index were used. The phase change is different for s and ppolarization. For each polarization, the phase change for a doubletransversal of layer i, b, is shown in Equations 9(a) and 9(b):$\begin{matrix}{\beta_{is} = {\frac{2\quad \pi \quad d_{i}}{\lambda}\sqrt{n_{ix}^{2} - {n_{o}^{2}\sin^{2}\theta_{o}}}\quad {\text{(for}\text{s}\text{polarized light)}}}} & \text{Equation~~9(a)}\end{matrix}$

$\begin{matrix}{\beta_{ip} = {\frac{2\quad \pi \quad {di}}{\gamma}\frac{n_{ix}}{n_{is}}\sqrt{n_{iz}^{2} - {n_{o}^{2}\sin^{2}\theta_{o}}}\quad {\text{(for}\text{p}\text{polarized light)}}}} & \text{Equation 9(b)}\end{matrix}$

where q_(o) and n_(o) are the angle and index of the incident medium.

Born & Wolf, in Principles of Optics, Pergamon Press 6th ed, 1980, p.66, showed that the wavelength edge of the high reflectance region canbe determined by evaluating the M₁₁ and M₂₂ elements of thecharacteristic matrix of the stack at different wavelengths. Atwavelengths where Equation 10 is satisfied, the transmissionexponentially decreases as more halfwaves are added to the stack.$\begin{matrix}{{\frac{M_{11} + M_{22}}{2}} \geq 1} & \text{Equation 10}\end{matrix}$

The wavelength where this expression equals 1 is the bandedge. For ahalfwave composed of two layers, multiplying the matrix results in theanalytical expression given in Equation 11. $\begin{matrix}{{\frac{M_{11} + M_{22}}{2}} = {{{{{\cos \left( \beta_{1} \right)}{\cos \left( \beta_{2} \right)}} - {\frac{1}{2}\left( {\frac{n_{ld}}{n_{lo}} + \frac{n_{lo}}{n_{ld}}} \right){\sin \left( \beta_{1} \right)}{\sin \left( \beta_{2} \right)}}}} \geq 1}} & \text{Equation 11}\end{matrix}$

The edge of a reflection band can be determined from the characteristicmatrix for each halfwave. For a halfwave with more than two layers, thecharacteristic matrix for the stack can be derived by matrixmultiplication of the component layers to generate the total matrix atany wavelength. A bandedge is defined by wavelengths where Equation 11is satisfied. This can,be either the first order reflection band orhigher order reflections. For each band, there are two solutions. Thereare additional solutions at shorter wavelengths where higher orderreflections can be found.

Manufacture

A preferred method of making the multilayer film of the presentinvention is illustrated schematically in FIG. 7, and is described ineven greater detail in U.S. Ser. No. 09/006288 entitled “Process forMaking Multilayer Optical Films” filed on even date and having AttorneyDocket No. 51932USA8A, the contents of which are incorporated herein byreference. To make multilayer optical films, materials 100 and 102selected to have suitably different optical properties are heated abovetheir melting and/or glass transition temperatures and fed into amultilayer feedblock 104, with or without a layer multiplier 106. Thelayer multiplier 106 splits the multilayer flow stream, and thenredirects and “stacks” one stream atop the second to multiply the numberof layers extruded. An asymmetric multiplier, when used with extrusionequipment that introduces layer thickness deviations throughout thestack, may broaden the distribution of layer thicknesses so as to enablethe multilayer film to have layer pairs corresponding to a desiredportion of the visible spectrum of light, and provide a desired layerthickness gradient. Skin layers may also be introduced by providingmaterial 108 for skin layers to a skin layer feedblock 110.

The multilayer feedblock feeds a film extrusion die 112. Feedblocksuseful in the manufacture of the present invention are described in, forexample, U.S. Pat. Nos. 3,773,882 and 3,884,606, and U.S. Ser. No.09/006288 entitled “Process for Making Multilayer Optical Films” filedon even date under Attorney Docket No. 51932USA8A, all of which herebyincorporated by reference. As an example, the extrusion temperature maybe approximately 295° C., and the feed rate approximately 10-150 kg/hourfor each material. It is desirable in most cases to have skin layers 111flowing on the upper and lower surfaces of the film as it goes throughfeedblock 110 and before it goes through die 112. These layers 111 serveto dissipate the large stress gradient found near the wall, leading tosmoother extrusion of the optical layers. Typical extrusion rates foreach skin layer 111 would be 2-50 kg/hr (1-40% of the total throughput).The skin material may be the same as one of the optical layers, or athird polymer.

After exiting die 112, the melt is cooled on a casting wheel 116, whichrotates past pinning wire 114. Pinning wire 114 pins the extrudate tocasting wheel 116. The film then travels through lengthwise orientationpull rolls 118, tenter oven 120, heat set portion 122 of tenter oven 120and then to wind up roll 124. To achieve a clear film over a broaderrange of angles, one need only make the film thicker by running thecasting wheel more slowly. This moves the low bandedge farther away fromthe edge of the visible spectrum (700 nm). In this way, the color shiftof the films of this invention may be adjusted for the desired colorshift. The film is oriented by stretching at ratios determined withreference to the desired optical and mechanical properties. Stretchratios of approximately 34 to 1 are preferred, although ratios as smallas 2 to 1 and as large as 6 to 1 may also be appropriate to a givenfilm. Stretch temperatures will depend on the type of birefringentpolymer used, but 20 to 33° C. (50 to 60° F.) above its glass transitiontemperature would generally be an appropriate range. The film istypically heat set in the last two zones of tenter oven 120 to impartthe maximum crystallinity in the film and reduce its shrinkage.Employing a heat set temperature as high as possible without causingfilm breakage in the tenter oven 120 reduces the shrinkage during aheated embossing step. A reduction in the width of the tenter rails byabout 1-4% also serves to reduce film shrinkage. If the film is not heatset, heat shrink properties are maximized, which may be desirable insome security packaging applications.

A suitable multilayer optical body may also be prepared using techniquessuch as spin coating, M., as described, for example, in Boese et al., J.Polym. Sci.: Part B, 30:1321 (1992) for birefringent polyimides, andvacuum deposition, e.g., as described by Zang et. al., Appl. Phys.Letters, 59:823 (1991) for crystalline organic compounds; the lattertechnique is particularly useful for certain combinations of crystallineorganic compounds and inorganic materials.

Orientation of the extruded film may be done by stretching individualsheets of the material in heated air. For economical production,stretching may be accomplished on a continuous basis in a standardlength orienter, i.e., lengthwise orientation pull rolls 118, tenteroven 120, or both. Economies of scale and line speeds of standardpolymer film production may be achieved thereby achieving manufacturingcosts that are substantially lower than costs associated withcommercially available absorptive polarizers.

Additional Layers and Features

In addition to the skin layer described above, which add physicalstrength to the film and reduce problems during processing, other layersand features of the inventive film may include slip agents, low adhesionbacksize materials, conductive coatings, antistatic, antireflective orantifogging coatings or films, barrier layers, flame retardants, UVstabilizers or protective layers, abrasion resistant materials,opticalcoatings, or substrates to improve the mechanical integrity orstrength of the film. Non-continuous layers may also be incorporatedinto the film to prevent tampering.

It may be desirable to add to one or more of the layers, one or moreinorganic or organic adjuvants such as an antioxidant, extrusion aid,heat stabilizer, ultraviolet ray absorber, nucleator, surface projectionforming agent, and the like in normal quantities so long as the additiondoes not substantially interfere with the performance of the presentinvention.

Lamination of two or more sheets together may be advantageous, toimprove reflectivity or to broaden the bandwidth, or to form a mirrorfrom two polarizers. Amorphous copolyesters are useful as laminatingmaterials. Exemplary amorphous copolyesters include those commerciallyavailable from Goodyear Tire and Rubber Co. of Akron₁ Ohio, under thetrade designations “VITEL Brand 3000” and “VITEL Brand 3300”. The choiceof laminating material is broad, with adhesion to the sheets 10, opticalclarity and exclusion of air being the primary guiding principles.

A characteristic of dielectric multilayer thin interference films isthat the reflected wavelength shifts to shorter wavelengths at higherangles of view. To ensure that the encroachment of the reflecting bandon the visible range of wavelengths is acceptable, the thicknesses ofthe individual layer pairs are increased such that their resonant firstorder wavelengths occur at a higher wavelength than would normally bedesired. This increase follows equation 11.

Since solar energy and some other light sources have significantemission in the near infrared, there may be a substantial loss of thefilm's efficiency if the gap which has been created is not filled.Furthermore, while the gap is generally filled off-angle by thewavelength shift, many industry standard tests for energy efficiency ofwindows only involve measurements at normal incidence.

Wavelength Gap Filler Component

The optical body of the present invention further include a wavelengthgap filler component in conjunction with the film described above. Thegap filler component functions to either absorb or reflect the infraredwavelengths that are not reflected by the film at normal angles becauseof the need to shift the reflective band of the film to higherwavelengths in order to minimize perceived color changes at non-normalincidence. Depending on the placement of the gap filler componentrelative to the film, the component may not function at non-normalangles because the reflective band shifts to lower wavelengths,preferably coinciding with the wavelength region of the absorption orreflection of the gap filler component. Suitable gap filler componentsinclude an infrared absorbing dye or pigment, an infrared absorbingglass, a trailing segment, a plurality of isotropic layers, orcombinations thereof. The gap filler component may be a part of thefilm, for example, as a trailing segment or a plurality of isotropiclayers coextruded with the film layers or as a dye or pigmentincorporated into one or more of the film layers. Alternatively, the gapfiller component may be a discrete part of the optical body of thepresent invention, i.e., separate from the film, that is attached, forexample, laminated thereto. Examples of this embodiment include a dye orpigment as a separate layer adhered to the film. The description of thegap filler as a part of the film and separate from the film is merelyexemplary. The gap filler component disclosed herein may be either be apart of the film or may be separate from the film depending on thecharacteristics of the component itself and the film with which it isbeing combined.

The film and the gap filler components are preferably combined such thatthe film is placed on a surface nearest the sun as practical because itis more efficient to reflect solar energy than to absorb it. In otherwords, where possible, it is preferable that the sun's rays firstencounter the film and then secondarily encounter the gap fillercomponent. In a multiple pane or two-ply windshield, the most preferableplacement for the film is the exterior nearest the sun, the nextpreferably position is between the panes or plies. The film may beplaced on the interior surface but this allows absorption of solar lightby the glass before the light reaches the film and absorption of part ofthe light reflected from the film. This embodiment may be preferablewhen considered from a UV protection standpoint, since it may bepreferable to position the film away from the sun, allowing componentswhich are less sensitive to UV to absorb this part of the light.

Examples of suitable infrared absorbing dyes include cyanine dyes asdescribed, for example, in U.S. Pat. No. 4,973,572, hereby incorporatedby reference, as well as bridged cyanine dyes and trinuclear cyaninedyes as described, for example, in U.S. Pat. No. 5,034,303, herebyincorporated by reference, merocyanine dyes as described, for example,in U.S. Pat. No. 4,950,640, hereby incorporated by reference,carbocyanine dyes (for example, 3,3′-diethyloxatricarbocyanine iodide,1,1′,3,3,3′,3′-hexamethylindotricarbocyanine perchlorate,1,1′,3,3,3′,3′-hexamethylindotricarbocyanine iodide,3,3′-diethylthiatricarbocyanine iodide, 3,3′-diethylthiatricarbocyanineperchlorate,1,1′,3,3,3′,3′-hexamethyl-4,4′,5,5′-dibenzo-2,2′-indotricarbocyanineperchlorate, all of which are commercially available from Kodak,Rochester, N.Y.), and phthalocyanine dyes as described, for example, inU.S. Pat. No. 4,788,128, hereby incorporated by reference; naphthalinedyes; metal complex dyes, for example, metal dithiolate dyes (forexample, nickel dithiolate dyes and, for example,bis[4-dimethylaminodithiobenzil] nickel, bis[dithiobenzil] nickel,bis[1,2-bis(n-butylthio)ethene-1,2-dithiol]nickel, bis[4,4′-dimethoxydithiobenzil] nickel, bis[dithiobenzil] platinum,bis[dithioacetyl] nickel) and metal dithiolene dyes (for example,nickeldithiolene dyes as described, for example, in U.S. Pat. No.5,036,040, hereby incorporated by reference); polymethine dyes such asbis(chalcogenopyrylo)polymethine dyes as described, for example, in U.S.Pat. No. 4,948,777, hereby incorporated by reference, bis(aminoaryl)polymethine dyes as described, for example, in U.S. Pat. No. 4,950,639,hereby incorporated by reference, indene-bridged polymethine dyes asdescribed, for example, in U.S. Pat. No. 5,019,480, hereby incorporatedby reference, and tetraaryl polymethine dyes; diphenylmethane dyes;triphenylmethane dyes; quinone dyes; azo dyes; ferrous complexes asdescribed, for example, in U.S. Pat. No. 4,912,083, hereby incorporatedby reference; squarylium dyes as described, for example, in U.S. Pat.No. 4,942,141, hereby incorporated by reference;chalcogenopyrylo-arylidene dyes as described, for example, in U.S. Pat.No. 4,948,776, hereby incorporated by reference; oxoindolizine dyes asdescribed, for example, in U.S. Pat. No. 4.948.778, hereby incorporatedby reference; anthraquinone and naphthoquinone derived dyes asdescribed, for example, in U.S. Pat. No. 4,952,552, hereby incorporatedby reference; pyrrocoline dyes as described, for example, in U.S. Pat.No. 5,196,393, hereby incorporated by reference; oxonol dyes asdescribed, for example, in U.S. Pat. No. 5,035,977, hereby incorporatedby reference; squaraine dyes such as chromylium squaraine dyes,thiopyrylium squaraine dyes as described, for example, in U.S. Pat. No.5,019,549, hereby incorporated by reference, and thiochromyliumsquaraine dyes; polyisothianaphthene dyes; indoaniline and azomethinedyes as described, for example, in U.S. Pat. No. 5,193,737, herebyincorporated by reference; indoaniline methide dyes; tetraarylariniumradical cation dyes and metallized quinoline indoaniline dyes.Squarylium dyes or squaraines are also described, for example, in U.S.Pat. No. 4,942,141 and U.S. Pat. No. 5,019,549, both of which are herebyincorporated by reference.

Commercially available phthalocyanine dyes include, for example, thoseavailable from Zeneca Corporation, Blackley, Manchester, England underthe trade designation “Projet Series” for example, “Projet 830NP”,“Projet 860 NP” and “Projet 900NP”.

Commercially available metal complex dyes include those available fromC.C. Scientific Products, Ft. Worth, Tex. 76120, for example,bis[4-dimethylaminodithiobenzil] nickel.

Additional suitable dyes include those described in Jurgen Fabian'sarticle entitled “Near Infrared Absorbing Dyes” Chem Rev, 1992,1197-1226 and “The Sigma Aldrich Handbook of Stains, Dyes andIndicators” by Floyd J. Green, Aldrich Chemical Company, Inc.,Milwaukee, Wis. ISBN 0-941633-22-5, 1991, both of which are herebyincorporated by reference. Useful near infrared absorbing dyes includethose from Epolin, Inc., Newark, N.J., for example, having the tradedesignations: Epolight III-57, Epolight III-1 17, Epolight V-79,Epolight V-138, Epolight V-129, Epolight V-99, Epolight V-1 30, EpolightV-149, Epolight IV-66, Epolight IV-62A, and Epolight III-189.

Suitable infrared absorbing pigments include cyanines, metal oxides andsquaraines. Suitable pigments include those described in U.S. Pat. No.5,215,838, incorporated herein by reference, such as metalphthalocyanines, for example, vanadyl phthalocyanine, chloroindiumphthalocyanine, titanyl phthalocyanine, chloroaluminum phthalocyanine,copper phthalocyanine, magnesium phthalocyanine, and the like;squaraines, such as hydroxy squaraine, and the like; as well as mixturesthereof. Exemplary copper pthalocyanine pigments include the pigmentcommercially available from BASF under the trade designation “6912”.Other exemplary infrared pigments include the metal oxide pigmentcommercially available from Heubach Langelsheim under the tradedesignation “Heucodor”.

Dyes or pigments useful in the present invention may be narrow-bandabsorbing, absorbing in the region of the spectrum not covered becauseof the position of the short wavelength bandedge of the optical body,for example, 700 to 850 nm, or may be broad band, absorbing oversubstantially all or all of the infrared region.

The dye or pigment can be applied to either surface of the film, in alayer of glass or polymer, such as polycarbonate or acrylic, laminatedto the film, or be present in at least one of the polymer layers of thefilm. From a solar energy standpoint, the dye is preferably on theinnermost surface of the film (i.e. toward the room interior and awayfrom the sun) so that when the sun is a high angle, the film reflectiveband shifts to lower wavelengths, essentially coinciding with thewavelength region of the dye. This is preferred because reflecting solarenergy away from the building is preferred to absorbing it.

The amount of dye or pigment used in the optical body of the presentinvention varies depending on the type of dye or pigment and/or the enduse application. Typically, when applied to the surface of the film, thedye or pigment is present on the surface at a concentration and coatingthickness suitable to accomplish the desired infrared absorption andvisible appearance. Typically, if the dye or pigment is within anadditional layer or within the multilayer optical body, theconcentration ranges from about 0.05 to about 0.5 weight %, based on thetotal weight of the optical body. In addition, when a pigment is used, asmall particle size typically is needed, for example, less than thewavelength of light. If the dyes are non-polar solvent soluble, the dyescan be coated or mixed in with solid plastic pellets and extruded if thedyes can withstand the heat of mixing and extrusion.

Examples of suitable infrared absorbing glasses include clear glasshaving a thickness generally ranging from about 3 to about 6 mm, such asarchitectural or automotive glass; blue glass; or green glass whichselectively absorb in the near infrared, i.e., about 700 to 1800 nm.

In the embodiments where blue or green glass is used, it is preferablethat the film of the present invention is located on the surface of theglass closest to the sun so that the film can reflect away the 850-1250nm wavelengths, allowing some of the infrared which is not reflected tobe absorbed by the glass. If it is not practical to place the film onthe exterior surface of a glass layer, for example, on the exterior of awindow of a building, it may be useful to place the film between panesof glass, rather than on the surface closest to the interior, in thecase of multiple pane windows, in order to minimize absorption.Preferably, the exterior layer (closest to the sun) has minimal infraredabsorbing properties so that the film is able to reflect light in theinfrared region before this light reaches the interior infraredabsorbing glass. In this embodiment, tile glass temperature would belower and less heat would enter the room due to re-radiation of absorbedlight. Additionally, the glass and/or film would be cooler which wouldreduce cracking of the glass due to thermal stress, a common problemwith heavily absorbing materials.

Infrared absorbing glass is available commercially from companiesincluding Pittsburgh Plate Glass (PPG), Guardian, Pilkington-LibbeyOwens Ford, Toledo, Ohio.

Generally a sharp band edge is desired in optical interference filmssuch as the infrared reflective films described herein. Sharp band edgescan be obtained from proper design of the layer thickness gradientthroughout the multilayer optical stack, as described in U.S. Ser. No.09/006085 entitled “Optical Film with Sharpened Bandedge” filed by oneven date under Attorney Docket No. 53545USA7A. Instead, a reflectivefilm of the present invention can be designed to include a trailingsegment to partially reflect infrared wavelengths in the gap regionwithout. producing strong color in the visible spectrum at non-normalangles. A trailing segment can be provided as a multilayer interferencefilm have layer thicknesses and refractive indices such that thereflectance in the gap region is relatively weak, for example, 50% andwhich may decrease so that transfer from high reflectance to lowreflectance of the multilayer film is gradual For example, a layergradient may provide a sharp bandedge above, for example, the 50%reflectance point and a trailing segment could be provided by additionallayers. For example, instead of providing a sharp edge, the last 30layers of a 200 layer stack could be of appropriate optical thicknessthat their first order reflection occurs in the range of about 800-850nm, the intensity of which increase from about 90% reflection at 850 nmto about 25% at 800 nm. The other 170 layers could provide, for example,about 90% reflection from about 850-1150 nm. Achieving the trailingsegment can be done in a number of ways, for example, by controlling thevolumetric feed of the individual layers. The trailing segment may beextruded with the multilayer film of the present invention or laminatedthereto.

Possible advantages of a trailing segment is that instead of an abrupttransition from no color to maximum color, the trailing segment providesa “softer” transition which may be more aesthetically acceptable andeasier to control from a process standpoint.

An isotropic multilayer film, as described above, could also be used tocover at least a portion of the wavelength gap. The optical body of thepresent invention may comprise the combination of any film describedherein and any gap filler component described herein, with the provisothat when a polymeric isotropic material is selected as the film, thegap filler component is not a polymeric isotropic material and when apolymeric isotropic material is selected as the gap filler component,the film is not selected to be a polymeric isotropic material.

Isotropic layers lose p-pol reflection intensity at oblique angles. If abirefringent film is used as the multilayer film, for example, PEN/PMMA,isotropic layers can be used to cover at least a portion of the gap.Accordingly, at oblique angles, the z-index matched reflectance bandwould shift into the gap and the reflectance from the isotropic layerswould shift to the visible but also decrease in p-pol intensity. S-polwould be masked or partially masked by the air/optical body surfacewhich would increase its reflectance at oblique angles. Exemplaryisotropic polymers include but are not limited to isotropic coPEN, PMMA,polycarbonates, styrene acrylonitriles, PETG, PCTG, styrenics,polyurethanes, polyolefins, and fluoropolymers. The isotropic film couldbe coextruded with the film of the present invention or laminated tothis film.

Gap filler components may be used in combination with the multilayerfilm of the present invention, for example, when each gap fillercomponent only absorbs or reflects in a portion of the gap to be filled.In addition, shifting the bandedge and, thus, creating the gap, alsoserves to create another, or second, gap in the infrared region atlonger wavelengths off angle. Therefore, it may be preferable to alsoinclude a component which fills this second gap region off angle. Thepresent invention also encompasses an optical body comprising abirefringent or isotropic dielectric multilayer film, as describedabove, in combination with a gap filler component which only fills thesecond gap in the infrared region at longer wavelengths off angle.Suitable gap filler components to fill this second gap include dyes,pigments, glasses, metals and multilayer films which absorb or reflectin the longer wavelengths of the infrared region, as described above.

Preferably, gap filler component (a) is situated such that light hitsthe multilayer film of the present invention before it hits the gapfiller component so that, then when the sun is at normal incidence, thegap filler absorbs light in the region of the gap. However, when the sunis at high angles, the film will shift to some of the same wavelengthsas the gap filler component and serve to reflect at least some of thelight in the region of the gap.

Optional Elements

A multilayered infrared optical body in accordance with the presentinvention may be combined with a transparent conductor to provide atransparent multilayer optical body having broader reflectivity. Inparticular, the transparent conductor provides good far infraredrefection (above about 2500 nm) although its reflectivity in the nearinfrared region of the spectrum generally is not as good throughout theregion of about 700 nm to about 2500 nm. The optical body of the presentinvention can be designed or “tuned” to provide the desired infraredreflection while still transmitting sufficient light to be transparent.

The transparent conductors useful in the present invention are thosethat reflect light in the far infrared region of the spectrum, and moreparticularly include those effective in efficiently partitioninginfrared light (above about 700 nm) from visible light (between about380 nm and about 700 nm). In other words, the transparent conductorpasses light in the wavelength region sensitive to the human eye whilerejecting light in the infrared region. Because both high visibletransmission and low near infrared transmission are desired, thereflective edge necessarily must be above about 700 nm, just out of thesensitivity of the human eye. Suitable transparent conductors areelectrically conductive materials that reflect well in the far infraredspectrum and include metals, metal alloys, and semiconductive metaloxide materials. Preferred metals include silver, gold, copper, andaluminum. Other metals, such as nickel, sodium, chromium, tin, andtitanium, may also be used, but they generally are not as effective inpartitioning infrared light from visible light. Silver is particularlypreferred since it can be applied in the form of a very thin film andoptically has a relatively high transmittance over the entire visiblelight region while also possessing the ability to reflect light oflonger wavelengths than visible light. Preferred semiconductive metaloxides include doped and undoped tin dioxide (SnO₂), zinc oxide (ZnO),and indium tin oxide (ITO) with the latter being particularly preferred.Preferred metal alloys include silver alloys, stainless steel, andinconel. Silver alloys, especially those containing at least 30 wt. %silver, are particularly preferred for the same reasons that silver ispreferred, but have the added advantage of improved durability, such asa silver alloy containing, in addition to silver, less than 50 wt. %gold and/or less than 20 wt. % copper. The transparent conductor maycomprise a single metal layer or a plurality of layers, each of whichmay contain one or more metals, metal alloys, and metal oxides.

Metals and metal alloys useful as transparent conductors in the presentinvention have electrical conductivities ranging between about 0.02mhos/sq. to about 1.0 mhos/sq., preferably between about 0.05 mhos/sq.to about 1.0 mhos/sq., and may be applied in a thickness from about 10nm to about 40 nm, preferably between about 12 nm to about 30 nm.Preferred semiconductive metal oxide layers have an electricalconductivity ranging between about 0.0001 mhos/sq. to about 0.1mhos/sq., preferably between about 0.001 mhos/sq. to about 0.1 mhos/sq.,and may be applied in a thickness from about 20 nm to about 200 nm,preferably from about 80 nm to about 120 nm. Where the transparentconductor is a metalized polymer or glass sheet laminated to themultilayered polymer film, the metal or metal alloy coatings on thesheet preferably have a thickness from about 10 nm to about 40 nm, whilemetal oxide coatings on the sheet preferably have a thickness from about20 nm to about 200 nm.

Although thin metal transparent conductors, such as silver, may besufficiently thin to have high visible transmissions, their reflectivityin the near infrared region between about 700 nm and about 1200 nm isnot as good as compared to the reflectivity that can be achieved in thatregion by the multilayered polymer films used in the present invention.In contrast, the multilayered polymer films described above have hightransmission of visible light and comparatively good reflection in thenear infrared region with relatively low to poor reflectivity in the farinfrared region. The multilayered polymer films are also generallycapable of providing a sharper transition between visible and infraredlight than the transparent conductors. Thus, the combination of themultilayered polymer film with the transparent conductor to form thetransparent multilayer optical body of the present invention providesbetter reflectivity throughout the entire infrared region while stilltransmitting visible light. In addition, antireflective coatings, whichare well known to those of ordinary skill in the art, may be coated overthe transparent conductor to increase the transmission of visible light.This includes, for example, an antireflective coating consisting of ametal, dielectric, metal stack with the individual layer thicknessescontrolled to provide the desired visible transmission. However, suchantireflective coatings are not required by the present invention toobtain the desired transmission of light in the visible spectrum.

In a two component film, for example, the bandwidth of this reflectivityin the infrared region, however, is also dependent upon color and/or thelevel of transmission desired in the visible range since overtones andthird order effects, which occur for first orders above about 1150 nm,will undesirably increase reflection in the visible spectrum as is wellknown to those of ordinary skill in this art. One way to avoidsignificantly impacting deleteriously the transmission of visible lightis to control the thicknesses of the individual layers in themultilayered polymer film as discussed above to limit the reflectionband in the near infrared spectrum to a preselected range, such asbetween 700 nm and about 1150 nm where the solar spectrum is moreintense than further out in the infrared spectra. In such an embodiment,the desired transmission in the visible spectrum will be maintained, andthe combination of transparent conductor and multilayered polymer filmwill reflect the desired amount of light, with the multilayer filmdominating the reflection in the near infrared from about 700 nm toabout 1150 nm, and the transparent conductor dominating the reflectionin the infrared spectrum above about 1150 nm. Other ways to achieve thisresult are also known in the art. See, for example, Alfred Thelen,“Multilayer Filters with Wide Transmittance Bands,” J. Opt. Soc. Am. 53(11), 1963, p. 1266, and Philip Baumeister, “Multilayer Reflections WithSuppressed Higher Order Reflection Peaks,” Applied Optics 31 (10), 1992,p. 1568, and U.S. Pat. Nos. RE 34,605 and 5,360,659 and U.S. Ser. No.09/006118 entitled “Multicomponent Optical Body” filed on even dateunder Attorney Docket No. 53543USA1A. In these other designs, whichsuppress additional orders, one may determine the maximum first orderbandwidth extent into the infrared without encroaching into the visibleblue region by an unsuppressed overtone.

The transparent conductor may be applied to the multilayered polymerfilms by conventional coating techniques well-known to those of ordinaryskill in this art, with the understanding that the resultingmultilayered optical body is transparent. Such known processes includepyrolysis, powder coating, vapor deposition, cathode sputtering, ionplating, and the like. Cathode sputtering and vapor deposition are oftenpreferred in view of the uniformity of structure and thickness that canbe obtained. Alternately, the transparent conductor may be a separatemetalized polymer or glass sheet that is laminated to the multilayeredpolymer film by means of a suitable adhesive, preferably a hot meltadhesive such as the VITEL 3300 adhesive from Shell Chemical Company,Akron, Ohio, or a pressure sensitive adhesive such as 90/10 IOA/AA and95/5 IOA/acrylamide acrylic PSAs from Minnesota Mining and ManufacturingCompany (3M), St. Paul, Minn.

The thickness of the transparent conductor applied to the multilayeredpolymer films to form the transparent multilayer optical body of thepresent invention can be selected to provide the desired reflectivity.In general, the thinner the metal layer, the more light in the visiblespectrum will be transmitted. However, because the electricalconductivity of the metal layer decreases as its thickness decreases,the amount of light reflected in the far infrared spectrum alsodecreases as the thickness of the metal layer decreases. Accordingly, byadjusting the thickness of the metal layer for any particular metal,metal alloy, or semiconductive metal oxide, the transparent conductorcan provide the desired balance between transmission of light in thevisible spectrum and reflection of light in the far infrared spectrum.Moreover, the thickness of the metal layer deposited on the multilayeredpolymer film can be monitored by measuring the metal layer's electricalconductivity.

Shading Coefficient

The optical body of the present invention generally has no perceivedcolor change with a change in viewing angle or angle of incidence ofimpinging light, and is preferably uncolored, and has a modest shadingcoefficient. The shading coefficient is the amount of solar energy thatenters a window as compared to that of a simple pane of clear glass, andcan be measured as follows:

The measured sample transmission spectra is multiplied by thesensitivity function of the human eye integrated over the visiblespectrum and is referred to as T_(lum). The measured sample reflection(R_(AM2)) and transmission spectra (T_(AM2)) are integrated over airmass 2 solar spectrum according to ASTM E903, “Standard Test For SolarAbsorbance, Reflectance, and Transmittance of Materials UsingIntegrating Spheres.” The dominant wavelength is the apparent color ofthe sample that is calculated with CIE techniques using Illuminant C andthe 100 observer according to ASTM E308, “Standard Test Method forComputing The Colors of Objects Using the CIE System.” The color purityis the saturation of the color, with 0% being white and 100% being apure color. The shading coefficient is calculated from the air mass 2integrated R and T spectra of the silveroated multilayered polymer filmby the following formula:

SC=T^(g) _(AM2)+f×(100−T^(g) _(AM2)−R^(g) _(AM2))

where f is the inward flowing fraction of the absorbed solar energy.

The lower the shading coefficient value, the lower the amount of solarheat entering a room. The gap filler component creates a lower shadingcoefficient at normal angles. The optical body of the present inventionpreferably has a shading coefficient of less than 0.6.

Uses of the Optical Body

The optical body of the present invention have desirable opticalproperties in that they reflect and/or absorb the desired amount oflight in the infrared region of the spectrum, and preferablytransmitting sufficient light in the visible region of the spectrum tobe transparent. Thus, the optical body of the present invention controlsthe amount of solar energy that passes through it preferably withoutsignificantly decreasing the intensity or changing the color of lightsensed by the human eye at any angle.

By keeping out light in the infrared region, the optical body of thepresent invention aids in reducing required cooling in summer.Consequently, the optical body can be used by applying it directly tothe surface of a glass or plastic substrate, such as an exterior windowin a building or the windshield or window of an automobile, truck oraircraft. It is also suitable for laminated glass and plastic articlesin which at least one transparent multilayer optical body is sandwichedbetween pairs of glass or plastic panes. Other uses would be apparent tothose of ordinary skill in this art where protection is desired frominfrared radiation while still obtaining substantial transparency tolight in the visible region of the spectrum, such as, for example,applying the transparent multilayer optical body of the presentinvention to the window in a door to a refrigerated display case.

When the transparent multilayer optical body of the present inventionare applied to a window in a house or automobile to reflect solar heat,such as during the summer, preferably the gap filler is next to theinterior surface of the window and the multilayered polymer film facesthe house or automobile interior. The outer surface of the multilayeredpolymer film may be covered by an abrasion resistant coating, as is wellknown in the art. Where it is desired to reflect radiant heat from theroom back into the room during colder weather, the transparent conductoris preferably positioned facing the room or automobile interior. Inaddition, a protective polyolefin film, such as, for example, apolypropylene film, may be used to cover the optical body, if lowemissivity is desired, to maintain the reflectance in the far infraredregion. Such constructions are well known to those of ordinary skill inthe art. If the multilayer optical body of the present invention areused on the exterior of such windows, durability of the optical body isa concern. Accordingly, a protective UV-stabilized polyester or acrylicfilm layer may be laminated directly to the optical body.

In order to more fully and clearly describe the present invention sothat those skilled in the art may better understand how to practice thepresent invention, the invention will now be described by way of thefollowing examples. The examples are intended to illustrate theinvention and should not be construed as limiting the inventiondisclosed and claimed herein in any manner.

EXAMPLES Example 1

99.87% by weight 0.56 intrinsic viscosity (IV) PEN commerciallyavailable from Eastman Chemical Co., Knoxville, Tenn. under the tradedesignation “PEN 19109” and 0.13% by weight of a phthalocyanine dyecommercially available from Zeneca Corp., Manchester, United Kingdom,under the trade designation “Pro-Jet 830NP” were mixed together andextruded, at a temperature of 555° F. (291° C.) in a 1¼“extrudercommercially available from Killion, Inc., Cedar Grove, N.J. into athree layer construction with the outer layers being the PEN:dye mixtureand the inner layer being 100%/o by weight PEN. All three layers wereapproximately 0.004 inches (0.001 mm).

Example 2

A 210 layer infrared film of PEN:PMMA which was approximately 85%transmissive in the visible wavelength region was coated with PMMA intoluene commercially available from Rohm & Haas under the tradedesignation “B48S” diluted further with additional toluene to 27%solids. The coating was applied with a #12 meyer bar commerciallyavailable from RD.S. Co., Webster, N.Y., and dried in an oven for 10minutes at 210F. (99° C.) to produce a 7 μm thick coating on both sidesof the film. Transmission and spectral ringing was measured; thetransmission was found to have increased to about 89% and the spectralringing near the bandedge was reduced as compared to the film without acoating.

Example 3

Four samples of a 224 layer multilayer infrared reflecting filmconsisting of PEN:PMMA layers were prepared. The first sample had nocoating, and the second, third and fourth samples was coated with a dyeat thickness of 2 μm, 3.5 μm, and 7.3 μm, respectively, on the surfaceof the outer PEN skin layer using the meier bar technique. The dye was aphthalocyanine dye commercially available from Zeneca Corp., Manchester,United Kingdom, under the trade designation “Pro-Jet 830NP” at aconcentration of 3% by weight.

Solar properties for each sample are provided below.

Shading Coefficient Visible Transmission Sample 1 0.74 85.4% Sample 20.65 78.3% Sample 3 0.61 74.4% Sample 4 0.56 67.3%

The shading coefficient is 1.15 (TST+0.27 (SA)), where TST is totalsolar transmission from 300-2500 nm and SA is solar absorption from300-2500 nm.

Since a lower value of shading coefficient means that less solar heatenters a room, this data show an improvement in performance using a dyein accordance with the present invention.

Other modifications and variations of the present invention are possiblein light of the above teachings. It is to be understood, however, thatchanges may be made in the particular embodiments described above whichare within the full intended scope of the invention as defined in theappended claims.

What is claimed is:
 1. An optical body comprising: (a) a multilayer filmhaving a reflecting band positioned to reflect infrared radiation of atleast one polarization at an incident angle normal to the film, saidreflecting band having a short wavelength bandedge λ_(a0) and longwavelength bandedge λ_(b0) at a normal incident angle, and a shortwavelength bandedge λ_(aθ) and long wavelength bandedge λ_(bθ) at amaximum usage angle θ, wherein λ_(aθ) is less than λ_(a0) and λ_(a0) isselectively positioned at a wavelength greater than about 700 nm; and(b) at least one component which at least partially absorbs or reflectsradiation in the wavelength region between λ_(aθ) and λ_(a0) at a normalangle of incidence.
 2. The optical body of claim 1 wherein saidmultilayer film is dielectric.
 3. The optical body of claim 1 wherein atleast one layer of said multilayer film comprises a metal or metaloxide.
 4. The optical body of claim 1 wherein said at least onecomponent also at least partially absorbs or reflects radiation in thewavelength region between λ_(bθ) and λ_(b0) at a maximum usage angle θ.5. The optical body of claim 1 further comprising another componentwhich at least partially absorbs or reflects radiation in the wavelengthregion between λ_(bθ) and λ_(b0) at a maximum usage angle θ.
 6. Theoptical body of claim 1 wherein said at least one component partiallyabsorbs or reflects at _(λ) _(aθ) or greater where λ_(aθ) is 700 nm orgreater at the maximum usage angle θ.
 7. The optical body of claim 1wherein said at least one component is separate from the film andcomprises a dye or pigment.
 8. The optical body of claim 1 wherein saidat least one component is part of the film and comprises a dye orpigment.
 9. The optical body of claim 1 wherein said at least onecomponent is a pigment comprising copper phthalocyanine.
 10. The opticalbody of claim 1, wherein each of the layers of the multilayer film isoptically isotropic.
 11. The optical body of claim 1, wherein at leastsome of the layers of the multilayer film are birefringent.
 12. Anoptical body for absorbing or reflecting radiation over a range ofwavelengths defined by a lower wavelength edge, the optical bodycomprising: (a) a multilayer film having a normal incidence reflectionband that is defined by reflection of radiation of at least onepolarization at an incident angle normal to the multilayer film, whereinthe normal incidence reflection band is within the range of wavelengths;and (b) at least one additional component that at least partiallyabsorbs normally incident radiation in a wavelength region between thenormal incidence reflection band of the multilayer film and the lowerwavelength edge of the range of wavelengths.
 13. The optical body ofclaim 12, wherein the at least one additional component is separate fromthe multilayer film and comprises a dye or pigment.
 14. The optical bodyof claim 12, wherein the at least one additional component is part ofthe multilayer film and comprises a dye or pigment.
 15. The optical bodyof claim 12, wherein said multilayer film is dielectric.
 16. The opticalbody of claim 12, wherein at least one layer of said multilayer filmcomprises a metal or metal oxide.
 17. The optical body of claim 16,wherein the metal or metal oxide at least partially absorbs normallyincident radiation in a wavelength region between the normal incidencereflection band of the multilayer film and the lower wavelength edge ofthe range of wavelengths.
 18. The optical body of claim 12, wherein theat least one additional component also absorbs or reflects light atwavelengths higher than the normal incidence reflection band of themultilayer film.
 19. The optical body of claim 12, wherein the lowerwavelength edge is an infrared wavelength.
 20. An optical body forabsorbing or reflecting infrared light, the optical body comprising: (a)a multilayer film having a normal incidence reflection band that isdefined by reflection of infrared radiation of at least one polarizationat an incident angle normal to the multilayer film; and (b) at least oneadditional component that at least partially absorbs or reflectsnormally incident radiation in a wavelength region between the normalincidence reflection band of the multilayer film and 700 nm.
 21. Anoptical body for absorbing or reflecting infrared light, the opticalbody comprising: a multilayer film comprising a) a first multilayersegment comprising a plurality of layers and having a reflecting bandthat reflects at least 90% of infrared light of one polarization over afirst wavelength range; and b) a trailing segment comprising a pluralityof layers, the trailing segment having a reflecting band that reflectslight in a second wavelength range that i) extends over at least 50 nm,ii) is adjacent to the first wavelength range, and iii) covers lowerwavelengths than the first wavelength range; wherein the multilayer filmreflects at least 90% of infrared light of the one polarization at theedge of the second wavelength range adjacent to the first wavelengthrange and reflects no more than 25% of light of the one polarization atthe opposite edge of the second wavelength range.
 22. The optical bodyof claim 21, wherein within the second wavelength range, the multilayerfilm reflects at least 90% of infrared light at the edge of the secondwavelength range adjacent to the first wavelength range and reflects nomore than 25% of light at the opposite edge of the second wavelengthrange.
 23. An optical body for absorbing or reflecting radiation over arange of wavelengths defined by a lower wavelength edge, the opticalbody comprising: (a) a first multilayer structure comprising a pluralityof birefringent layers, the first multilayer structure having a normalincidence reflection band that is defined by reflection of radiation ofat least one polarization at an incident angle normal to the firstmultilayer structure, wherein the normal incidence reflection band iswithin the range of wavelengths; and (b) a second multilayer structureconsisting essentially of optically isotropic materials, wherein thesecond multilayer structure at least partially reflects normallyincident radiation in a wavelength region between the normal incidencereflection band of the first multilayer structure and the lowerwavelength edge of the range of wavelengths.
 24. The optical body ofclaim 23, wherein the first multilayer structure further comprises aplurality of optically isotropic layers.